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

@@ -47,6 +47,9 b' configitem = registrar.configitem(config'
 configitem('experimental', 'uncommitondirtywdir',
     default=False,
 )
+configitem('experimental', 'uncommit.keep',
+    default=False,
+)
 
 stringio = util.stringio
 
@@ -240,7 +243,7 b' def _uncommitdirstate(repo, oldctx, matc'
 
 @command('uncommit',
     [('i', 'interactive', False, _('interactive mode to 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)
@@ -270,7 +273,12 b' def uncommit(ui, repo, *pats, **opts):'
 
         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 interactive:
                 match = scmutil.match(old, pats, opts)

tests/test-uncommit.t

@@ -308,7 +308,7 b' Partial uncommit with public parent'
   $ 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
@@ -328,9 +328,31 b' Uncommit leaving an empty changeset'
   |/
   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