uncommit: add config option to keep commit by default

We have a Google-internal extension that keeps track of "review units"
(like Phabricator reviews, or Gerrit's Change-Id). This information is
stored outside of the commit. It is updated with rewrites. Every now
and then we get reports from users who are confused because `hg
uncommit` lost track of their review. Keeping the empty commit by
default would reduce this confusion. It may also cause confusion about
the empty commit. This patch adds a config option that lets us easily
test both behaviors on our users.

Differential Revision: https://phab.mercurial-scm.org/D5970

hgext/uncommit.py

@@ -1,248 +1,256 b''
 # uncommit - undo the actions of a commit
 #
 # Copyright 2011 Peter Arrenbrecht <[e-mail]>
 #                Logilab SA        <[e-mail]>
 #                Pierre-Yves David <[e-mail]>
 #                Patrick Mezard <[e-mail]>
 # Copyright 2016 Facebook, Inc.
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """uncommit part or all of a local changeset (EXPERIMENTAL)
 
 This command undoes the effect of a local commit, returning the affected
 files to their uncommitted state. This means that files modified, added or
 removed in the changeset will be left unchanged, and so will remain modified,
 added and removed in the working directory.
 """
 
 from __future__ import absolute_import
 
 from mercurial.i18n import _
 
 from mercurial import (
     cmdutil,
     commands,
     context,
     copies as copiesmod,
     error,
     node,
     obsutil,
     pycompat,
     registrar,
     rewriteutil,
     scmutil,
 )
 
 cmdtable = {}
 command = registrar.command(cmdtable)
 
 configtable = {}
 configitem = registrar.configitem(configtable)
 
 configitem('experimental', 'uncommitondirtywdir',
     default=False,
 )
+configitem('experimental', 'uncommit.keep',
+    default=False,
+)
 
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
 # leave the attribute unspecified.
 testedwith = 'ships-with-hg-core'
 
 def _commitfiltered(repo, ctx, match, keepcommit):
     """Recommit ctx with changed files not in match. Return the new
     node identifier, or None if nothing changed.
     """
     base = ctx.p1()
     # ctx
     initialfiles = set(ctx.files())
     exclude = set(f for f in initialfiles if match(f))
 
     # No files matched commit, so nothing excluded
     if not exclude:
         return None
 
     # return the p1 so that we don't create an obsmarker later
     if not keepcommit:
         return ctx.p1().node()
 
     files = (initialfiles - exclude)
     # Filter copies
     copied = copiesmod.pathcopies(base, ctx)
     copied = dict((dst, src) for dst, src in copied.iteritems()
                   if dst in files)
     def filectxfn(repo, memctx, path, contentctx=ctx, redirect=()):
         if path not in contentctx:
             return None
         fctx = contentctx[path]
         mctx = context.memfilectx(repo, memctx, fctx.path(), fctx.data(),
                                   fctx.islink(),
                                   fctx.isexec(),
                                   copied=copied.get(path))
         return mctx
 
     if not files:
         repo.ui.status(_("note: keeping empty commit\n"))
 
     new = context.memctx(repo,
                          parents=[base.node(), node.nullid],
                          text=ctx.description(),
                          files=files,
                          filectxfn=filectxfn,
                          user=ctx.user(),
                          date=ctx.date(),
                          extra=ctx.extra())
     return repo.commitctx(new)
 
 def _fixdirstate(repo, oldctx, newctx, match=None):
     """ fix the dirstate after switching the working directory from oldctx to
     newctx which can be result of either unamend or uncommit.
     """
     ds = repo.dirstate
     ds.setparents(newctx.node(), node.nullid)
     copies = dict(ds.copies())
     s = newctx.status(oldctx, match=match)
     for f in s.modified:
         if ds[f] == 'r':
             # modified + removed -> removed
             continue
         ds.normallookup(f)
 
     for f in s.added:
         if ds[f] == 'r':
             # added + removed -> unknown
             ds.drop(f)
         elif ds[f] != 'a':
             ds.add(f)
 
     for f in s.removed:
         if ds[f] == 'a':
             # removed + added -> normal
             ds.normallookup(f)
         elif ds[f] != 'r':
             ds.remove(f)
 
     # Merge old parent and old working dir copies
     oldcopies = copiesmod.pathcopies(newctx, oldctx, match)
     oldcopies.update(copies)
     copies = dict((dst, oldcopies.get(src, src))
                   for dst, src in oldcopies.iteritems())
     # Adjust the dirstate copies
     for dst, src in copies.iteritems():
         if (src not in newctx or dst in newctx or ds[dst] != 'a'):
             src = None
         ds.copy(src, dst)
 
 @command('uncommit',
-    [('', 'keep', False, _('allow an empty commit after uncommiting')),
+    [('', 'keep', None, _('allow an empty commit after uncommiting')),
     ] + commands.walkopts,
     _('[OPTION]... [FILE]...'),
     helpcategory=command.CATEGORY_CHANGE_MANAGEMENT)
 def uncommit(ui, repo, *pats, **opts):
     """uncommit part or all of a local changeset
 
     This command undoes the effect of a local commit, returning the affected
     files to their uncommitted state. This means that files modified or
     deleted in the changeset will be left unchanged, and so will remain
     modified in the working directory.
 
     If no files are specified, the commit will be pruned, unless --keep is
     given.
     """
     opts = pycompat.byteskwargs(opts)
 
     with repo.wlock(), repo.lock():
 
         if not pats and not repo.ui.configbool('experimental',
                                                'uncommitondirtywdir'):
             cmdutil.bailifchanged(repo)
         old = repo['.']
         rewriteutil.precheck(repo, [old.rev()], 'uncommit')
         if len(old.parents()) > 1:
             raise error.Abort(_("cannot uncommit merge changeset"))
 
         with repo.transaction('uncommit'):
             match = scmutil.match(old, pats, opts)
-            keepcommit = opts.get('keep') or pats
+            keepcommit = pats
+            if not keepcommit:
+                if opts.get('keep') is not None:
+                    keepcommit = opts.get('keep')
+                else:
+                    keepcommit = ui.configbool('experimental', 'uncommit.keep')
             newid = _commitfiltered(repo, old, match, keepcommit)
             if newid is None:
                 ui.status(_("nothing to uncommit\n"))
                 return 1
 
             mapping = {}
             if newid != old.p1().node():
                 # Move local changes on filtered changeset
                 mapping[old.node()] = (newid,)
             else:
                 # Fully removed the old commit
                 mapping[old.node()] = ()
 
             with repo.dirstate.parentchange():
                 _fixdirstate(repo, old, repo[newid], match)
 
             scmutil.cleanupnodes(repo, mapping, 'uncommit', fixphase=True)
 
 def predecessormarkers(ctx):
     """yields the obsolete markers marking the given changeset as a successor"""
     for data in ctx.repo().obsstore.predecessors.get(ctx.node(), ()):
         yield obsutil.marker(ctx.repo(), data)
 
 @command('unamend', [], helpcategory=command.CATEGORY_CHANGE_MANAGEMENT,
          helpbasic=True)
 def unamend(ui, repo, **opts):
     """undo the most recent amend operation on a current changeset
 
     This command will roll back to the previous version of a changeset,
     leaving working directory in state in which it was before running
     `hg amend` (e.g. files modified as part of an amend will be
     marked as modified `hg status`)
     """
 
     unfi = repo.unfiltered()
     with repo.wlock(), repo.lock(), repo.transaction('unamend'):
 
         # identify the commit from which to unamend
         curctx = repo['.']
 
         rewriteutil.precheck(repo, [curctx.rev()], 'unamend')
 
         # identify the commit to which to unamend
         markers = list(predecessormarkers(curctx))
         if len(markers) != 1:
             e = _("changeset must have one predecessor, found %i predecessors")
             raise error.Abort(e % len(markers))
 
         prednode = markers[0].prednode()
         predctx = unfi[prednode]
 
         # add an extra so that we get a new hash
         # note: allowing unamend to undo an unamend is an intentional feature
         extras = predctx.extra()
         extras['unamend_source'] = curctx.hex()
 
         def filectxfn(repo, ctx_, path):
             try:
                 return predctx.filectx(path)
             except KeyError:
                 return None
 
         # Make a new commit same as predctx
         newctx = context.memctx(repo,
                                 parents=(predctx.p1(), predctx.p2()),
                                 text=predctx.description(),
                                 files=predctx.files(),
                                 filectxfn=filectxfn,
                                 user=predctx.user(),
                                 date=predctx.date(),
                                 extra=extras)
         newprednode = repo.commitctx(newctx)
         newpredctx = repo[newprednode]
         dirstate = repo.dirstate
 
         with dirstate.parentchange():
             _fixdirstate(repo, curctx, newpredctx)
 
         mapping = {curctx.node(): (newprednode,)}
         scmutil.cleanupnodes(repo, mapping, 'unamend', fixphase=True)

tests/test-uncommit.t

@@ -1,443 +1,465 b''
 Test uncommit - set up the config
 
   $ cat >> $HGRCPATH <<EOF
   > [experimental]
   > evolution.createmarkers=True
   > evolution.allowunstable=True
   > [extensions]
   > uncommit =
   > drawdag=$TESTDIR/drawdag.py
   > EOF
 
 Build up a repo
 
   $ hg init repo
   $ cd repo
   $ hg bookmark foo
 
 Help for uncommit
 
   $ hg help uncommit
   hg uncommit [OPTION]... [FILE]...
   
   uncommit part or all of a local changeset
   
       This command undoes the effect of a local commit, returning the affected
       files to their uncommitted state. This means that files modified or
       deleted in the changeset will be left unchanged, and so will remain
       modified in the working directory.
   
       If no files are specified, the commit will be pruned, unless --keep is
       given.
   
   (use 'hg help -e uncommit' to show help for the uncommit extension)
   
   options ([+] can be repeated):
   
       --keep                allow an empty commit after uncommiting
    -I --include PATTERN [+] include names matching the given patterns
    -X --exclude PATTERN [+] exclude names matching the given patterns
   
   (some details hidden, use --verbose to show complete help)
 
 Uncommit with no commits should fail
 
   $ hg uncommit
   abort: cannot uncommit null changeset
   (no changeset checked out)
   [255]
 
 Create some commits
 
   $ touch files
   $ hg add files
   $ for i in a ab abc abcd abcde; do echo $i > files; echo $i > file-$i; hg add file-$i; hg commit -m "added file-$i"; done
   $ ls
   file-a
   file-ab
   file-abc
   file-abcd
   file-abcde
   files
 
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   @  4:6c4fd43ed714e7fcd8adbaa7b16c953c2e985b60 added file-abcde
   |
   o  3:6db330d65db434145c0b59d291853e9a84719b24 added file-abcd
   |
   o  2:abf2df566fc193b3ac34d946e63c1583e4d4732b added file-abc
   |
   o  1:69a232e754b08d568c4899475faf2eb44b857802 added file-ab
   |
   o  0:3004d2d9b50883c1538fc754a3aeb55f1b4084f6 added file-a
   
 Simple uncommit off the top, also moves bookmark
 
   $ hg bookmark
    * foo                       4:6c4fd43ed714
   $ hg uncommit
   $ hg status
   M files
   A file-abcde
   $ hg bookmark
    * foo                       3:6db330d65db4
 
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   x  4:6c4fd43ed714e7fcd8adbaa7b16c953c2e985b60 added file-abcde
   |
   @  3:6db330d65db434145c0b59d291853e9a84719b24 added file-abcd
   |
   o  2:abf2df566fc193b3ac34d946e63c1583e4d4732b added file-abc
   |
   o  1:69a232e754b08d568c4899475faf2eb44b857802 added file-ab
   |
   o  0:3004d2d9b50883c1538fc754a3aeb55f1b4084f6 added file-a
   
 
 Recommit
 
   $ hg commit -m 'new change abcde'
   $ hg status
   $ hg heads -T '{rev}:{node} {desc}'
   5:0c07a3ccda771b25f1cb1edbd02e683723344ef1 new change abcde (no-eol)
 
 Uncommit of non-existent and unchanged files has no effect
   $ hg uncommit nothinghere
   nothing to uncommit
   [1]
   $ hg status
   $ hg uncommit file-abc
   nothing to uncommit
   [1]
   $ hg status
 
 Try partial uncommit, also moves bookmark
 
   $ hg bookmark
    * foo                       5:0c07a3ccda77
   $ hg uncommit files
   $ hg status
   M files
   $ hg bookmark
    * foo                       6:3727deee06f7
   $ hg heads -T '{rev}:{node} {desc}'
   6:3727deee06f72f5ffa8db792ee299cf39e3e190b new change abcde (no-eol)
   $ hg log -r . -p -T '{rev}:{node} {desc}'
   6:3727deee06f72f5ffa8db792ee299cf39e3e190b new change abcdediff -r 6db330d65db4 -r 3727deee06f7 file-abcde
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   +++ b/file-abcde	Thu Jan 01 00:00:00 1970 +0000
   @@ -0,0 +1,1 @@
   +abcde
   
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   @  6:3727deee06f72f5ffa8db792ee299cf39e3e190b new change abcde
   |
   | x  5:0c07a3ccda771b25f1cb1edbd02e683723344ef1 new change abcde
   |/
   | x  4:6c4fd43ed714e7fcd8adbaa7b16c953c2e985b60 added file-abcde
   |/
   o  3:6db330d65db434145c0b59d291853e9a84719b24 added file-abcd
   |
   o  2:abf2df566fc193b3ac34d946e63c1583e4d4732b added file-abc
   |
   o  1:69a232e754b08d568c4899475faf2eb44b857802 added file-ab
   |
   o  0:3004d2d9b50883c1538fc754a3aeb55f1b4084f6 added file-a
   
   $ hg commit -m 'update files for abcde'
 
 Uncommit with dirty state
 
   $ echo "foo" >> files
   $ cat files
   abcde
   foo
   $ hg status
   M files
   $ hg uncommit
   abort: uncommitted changes
   [255]
   $ hg uncommit files
   note: keeping empty commit
   $ cat files
   abcde
   foo
   $ hg commit --amend -m "files abcde + foo"
 
 Testing the 'experimental.uncommitondirtywdir' config
 
   $ echo "bar" >> files
   $ hg uncommit
   abort: uncommitted changes
   [255]
   $ hg uncommit --config experimental.uncommitondirtywdir=True
   $ hg commit -m "files abcde + foo"
 
 Uncommit in the middle of a stack, does not move bookmark
 
   $ hg checkout '.^^^'
   1 files updated, 0 files merged, 2 files removed, 0 files unresolved
   (leaving bookmark foo)
   $ hg log -r . -p -T '{rev}:{node} {desc}'
   2:abf2df566fc193b3ac34d946e63c1583e4d4732b added file-abcdiff -r 69a232e754b0 -r abf2df566fc1 file-abc
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   +++ b/file-abc	Thu Jan 01 00:00:00 1970 +0000
   @@ -0,0 +1,1 @@
   +abc
   diff -r 69a232e754b0 -r abf2df566fc1 files
   --- a/files	Thu Jan 01 00:00:00 1970 +0000
   +++ b/files	Thu Jan 01 00:00:00 1970 +0000
   @@ -1,1 +1,1 @@
   -ab
   +abc
   
   $ hg bookmark
      foo                       10:48e5bd7cd583
   $ hg uncommit
   3 new orphan changesets
   $ hg status
   M files
   A file-abc
   $ hg heads -T '{rev}:{node} {desc}'
   10:48e5bd7cd583eb24164ef8b89185819c84c96ed7 files abcde + foo (no-eol)
   $ hg bookmark
      foo                       10:48e5bd7cd583
   $ hg commit -m 'new abc'
   created new head
 
 Partial uncommit in the middle, does not move bookmark
 
   $ hg checkout '.^'
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg log -r . -p -T '{rev}:{node} {desc}'
   1:69a232e754b08d568c4899475faf2eb44b857802 added file-abdiff -r 3004d2d9b508 -r 69a232e754b0 file-ab
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   +++ b/file-ab	Thu Jan 01 00:00:00 1970 +0000
   @@ -0,0 +1,1 @@
   +ab
   diff -r 3004d2d9b508 -r 69a232e754b0 files
   --- a/files	Thu Jan 01 00:00:00 1970 +0000
   +++ b/files	Thu Jan 01 00:00:00 1970 +0000
   @@ -1,1 +1,1 @@
   -a
   +ab
   
   $ hg bookmark
      foo                       10:48e5bd7cd583
   $ hg uncommit file-ab
   1 new orphan changesets
   $ hg status
   A file-ab
 
   $ hg heads -T '{rev}:{node} {desc}\n'
   12:8eb87968f2edb7f27f27fe676316e179de65fff6 added file-ab
   11:5dc89ca4486f8a88716c5797fa9f498d13d7c2e1 new abc
   10:48e5bd7cd583eb24164ef8b89185819c84c96ed7 files abcde + foo
 
   $ hg bookmark
      foo                       10:48e5bd7cd583
   $ hg commit -m 'update ab'
   $ hg status
   $ hg heads -T '{rev}:{node} {desc}\n'
   13:f21039c59242b085491bb58f591afc4ed1c04c09 update ab
   11:5dc89ca4486f8a88716c5797fa9f498d13d7c2e1 new abc
   10:48e5bd7cd583eb24164ef8b89185819c84c96ed7 files abcde + foo
 
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   @  13:f21039c59242b085491bb58f591afc4ed1c04c09 update ab
   |
   o  12:8eb87968f2edb7f27f27fe676316e179de65fff6 added file-ab
   |
   | *  11:5dc89ca4486f8a88716c5797fa9f498d13d7c2e1 new abc
   | |
   | | *  10:48e5bd7cd583eb24164ef8b89185819c84c96ed7 files abcde + foo
   | | |
   | | | x  9:8a6b58c173ca6a2e3745d8bd86698718d664bc6c files abcde + foo
   | | |/
   | | | x  8:39ad452c7f684a55d161c574340c5766c4569278 update files for abcde
   | | |/
   | | | x  7:0977fa602c2fd7d8427ed4e7ee15ea13b84c9173 update files for abcde
   | | |/
   | | *  6:3727deee06f72f5ffa8db792ee299cf39e3e190b new change abcde
   | | |
   | | | x  5:0c07a3ccda771b25f1cb1edbd02e683723344ef1 new change abcde
   | | |/
   | | | x  4:6c4fd43ed714e7fcd8adbaa7b16c953c2e985b60 added file-abcde
   | | |/
   | | *  3:6db330d65db434145c0b59d291853e9a84719b24 added file-abcd
   | | |
   | | x  2:abf2df566fc193b3ac34d946e63c1583e4d4732b added file-abc
   | |/
   | x  1:69a232e754b08d568c4899475faf2eb44b857802 added file-ab
   |/
   o  0:3004d2d9b50883c1538fc754a3aeb55f1b4084f6 added file-a
   
 Uncommit with draft parent
 
   $ hg uncommit
   $ hg phase -r .
   12: draft
   $ hg commit -m 'update ab again'
 
 Phase is preserved
 
   $ hg uncommit --keep --config phases.new-commit=secret
   note: keeping empty commit
   $ hg phase -r .
   15: draft
   $ hg commit --amend -m 'update ab again'
 
 Uncommit with public parent
 
   $ hg phase -p "::.^"
   $ hg uncommit
   $ hg phase -r .
   12: public
 
 Partial uncommit with public parent
 
   $ echo xyz > xyz
   $ hg add xyz
   $ hg commit -m "update ab and add xyz"
   $ hg uncommit xyz
   $ hg status
   A xyz
   $ hg phase -r .
   18: draft
   $ hg phase -r ".^"
   12: public
 
-Uncommit leaving an empty changeset
+Uncommit with --keep or experimental.uncommit.keep leaves an empty changeset
 
   $ cd $TESTTMP
   $ hg init repo1
   $ cd repo1
   $ hg debugdrawdag <<'EOS'
   > Q
   > |
   > P
   > EOS
   $ hg up Q -q
   $ hg uncommit --keep
   note: keeping empty commit
   $ hg log -G -T '{desc} FILES: {files}'
   @  Q FILES:
   |
   | x  Q FILES: Q
   |/
   o  P FILES: P
   
+  $ cat >> .hg/hgrc <<EOF
+  > [experimental]
+  > uncommit.keep=True
+  > EOF
+  $ hg ci --amend
+  $ hg uncommit
+  note: keeping empty commit
+  $ hg log -G -T '{desc} FILES: {files}'
+  @  Q FILES:
+  |
+  | x  Q FILES: Q
+  |/
+  o  P FILES: P
+  
   $ hg status
   A Q
-
+  $ hg ci --amend
+  $ hg uncommit --no-keep
+  $ hg log -G -T '{desc} FILES: {files}'
+  x  Q FILES: Q
+  |
+  @  P FILES: P
+  
+  $ hg status
+  A Q
   $ cd ..
   $ rm -rf repo1
 
 Testing uncommit while merge
 
   $ hg init repo2
   $ cd repo2
 
 Create some history
 
   $ touch a
   $ hg add a
   $ for i in 1 2 3; do echo $i > a; hg commit -m "a $i"; done
   $ hg checkout 0
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ touch b
   $ hg add b
   $ for i in 1 2 3; do echo $i > b; hg commit -m "b $i"; done
   created new head
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   @  5:2cd56cdde163ded2fbb16ba2f918c96046ab0bf2 b 3
   |
   o  4:c3a0d5bb3b15834ffd2ef9ef603e93ec65cf2037 b 2
   |
   o  3:49bb009ca26078726b8870f1edb29fae8f7618f5 b 1
   |
   | o  2:990982b7384266e691f1bc08ca36177adcd1c8a9 a 3
   | |
   | o  1:24d38e3cf160c7b6f5ffe82179332229886a6d34 a 2
   |/
   o  0:ea4e33293d4d274a2ba73150733c2612231f398c a 1
   
 
 Add and expect uncommit to fail on both merge working dir and merge changeset
 
   $ hg merge 2
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
   $ hg uncommit
   abort: outstanding uncommitted merge
   [255]
 
   $ hg uncommit --config experimental.uncommitondirtywdir=True
   abort: cannot uncommit while merging
   [255]
 
   $ hg status
   M a
   $ hg commit -m 'merge a and b'
 
   $ hg uncommit
   abort: cannot uncommit merge changeset
   [255]
 
   $ hg status
   $ hg log -G -T '{rev}:{node} {desc}' --hidden
   @    6:c03b9c37bc67bf504d4912061cfb527b47a63c6e merge a and b
   |\
   | o  5:2cd56cdde163ded2fbb16ba2f918c96046ab0bf2 b 3
   | |
   | o  4:c3a0d5bb3b15834ffd2ef9ef603e93ec65cf2037 b 2
   | |
   | o  3:49bb009ca26078726b8870f1edb29fae8f7618f5 b 1
   | |
   o |  2:990982b7384266e691f1bc08ca36177adcd1c8a9 a 3
   | |
   o |  1:24d38e3cf160c7b6f5ffe82179332229886a6d34 a 2
   |/
   o  0:ea4e33293d4d274a2ba73150733c2612231f398c a 1
   
 
 Rename a->b, then remove b in working copy. Result should remove a.
 
   $ hg co -q 0
   $ hg mv a b
   $ hg ci -qm 'move a to b'
   $ hg rm b
   $ hg uncommit --config experimental.uncommitondirtywdir=True
   $ hg st --copies
   R a
   $ hg revert a
 
 Rename a->b, then rename b->c in working copy. Result should rename a->c.
 
   $ hg co -q 0
   $ hg mv a b
   $ hg ci -qm 'move a to b'
   $ hg mv b c
   $ hg uncommit --config experimental.uncommitondirtywdir=True
   $ hg st --copies
   A c
     a
   R a
   $ hg revert a
   $ hg forget c
   $ rm c
 
 Copy a->b1 and a->b2, then rename b1->c in working copy. Result should copy a->b2 and a->c.
 
   $ hg co -q 0
   $ hg cp a b1
   $ hg cp a b2
   $ hg ci -qm 'move a to b1 and b2'
   $ hg mv b1 c
   $ hg uncommit --config experimental.uncommitondirtywdir=True
   $ hg st --copies
   A b2
     a
   A c
     a