merge: add options to warn or ignore on colliding unknown files

A 'colliding unknown file' is a file that meets all of the following
conditions:

- is untracked or ignored on disk
- is present in the changeset being merged or updated to
- has different contents

Previously, we would always abort whenever we saw such files. With this config
option we can choose to warn and back the unknown files up instead, or even
forgo the warning entirely and silently back the unknown files up.

Common use cases for this configuration include a large scale transition of
formerly ignored unknown files to tracked files. In some cases the files can be
given new names, but in other cases, external "convention over configuration"
constraints have determined that the file must retain the same name as before.

mercurial/help/config.txt

@@ -987,6 +987,19 b' proxy.'
     Optional. Always use the proxy, even for localhost and any entries
     in ``http_proxy.no``. (default: False)
 
+``merge``
+---------
+
+This section specifies behavior during merges and updates.
+
+``checkunknown``
+   Controls behavior when an unknown file on disk has the same name as a tracked
+   file in the changeset being merged or updated to, and has different
+   contents. Options are ``abort``, ``warn`` and ``ignore``. With ``abort``,
+   abort on such files. With ``warn``, warn on such files and back them up as
+   .orig. With ``ignore``, don't print a warning and back them up as
+   .orig. (default: ``abort``)
+
 ``merge-patterns``
 ------------------
 

mercurial/merge.py

@@ -573,6 +573,14 b' def _checkunknownfiles(repo, wctx, mctx,'
     """
     conflicts = set()
     if not force:
+        config = repo.ui.config('merge', 'checkunknown', default='abort')
+        valid = ['abort', 'ignore', 'warn']
+        if config not in valid:
+            validstr = ', '.join(["'" + v + "'" for v in valid])
+            raise error.ConfigError(_("merge.checkunknown not valid "
+                                      "('%s' is none of %s)")
+                                    % (config, validstr))
+
         for f, (m, args, msg) in actions.iteritems():
             if m in ('c', 'dc'):
                 if _checkunknownfile(repo, wctx, mctx, f):
@@ -581,16 +589,21 b' def _checkunknownfiles(repo, wctx, mctx,'
                 if _checkunknownfile(repo, wctx, mctx, f, args[0]):
                     conflicts.add(f)
 
+        if config == 'abort':
         for f in sorted(conflicts):
             repo.ui.warn(_("%s: untracked file differs\n") % f)
         if conflicts:
-            raise error.Abort(_("untracked files in working directory differ "
-                                "from files in requested revision"))
+                raise error.Abort(_("untracked files in working directory "
+                                    "differ from files in requested revision"))
+        elif config == 'warn':
+            for f in sorted(conflicts):
+                repo.ui.warn(_("%s: replacing untracked file\n") % f)
 
     for f, (m, args, msg) in actions.iteritems():
+        backup = f in conflicts
         if m == 'c':
             flags, = args
-            actions[f] = ('g', (flags, False), msg)
+            actions[f] = ('g', (flags, backup), msg)
         elif m == 'cm':
             fl2, anc = args
             different = _checkunknownfile(repo, wctx, mctx, f)
@@ -598,7 +611,7 b' def _checkunknownfiles(repo, wctx, mctx,'
                 actions[f] = ('m', (f, f, None, False, anc),
                               "remote differs from untracked local")
             else:
-                actions[f] = ('g', (fl2, False), "remote created")
+                actions[f] = ('g', (fl2, backup), "remote created")
 
 def _forgetremoved(wctx, mctx, branchmerge):
     """

tests/test-merge1.t

@@ -124,7 +124,43 b" symlinks shouldn't be followed"
   $ echo This is file b2 > b
 #endif
 
-merge of b expected
+bad config
+  $ hg merge 1 --config merge.checkunknown=x
+  abort: merge.checkunknown not valid ('x' is none of 'abort', 'ignore', 'warn')
+  [255]
+this merge should fail
+  $ hg merge 1 --config merge.checkunknown=abort
+  b: untracked file differs
+  abort: untracked files in working directory differ from files in requested revision
+  [255]
+
+this merge should warn
+  $ hg merge 1 --config merge.checkunknown=warn
+  b: replacing untracked file
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat b.orig
+  This is file b2
+  $ hg up --clean 2
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mv b.orig b
+
+this merge should silently ignore
+  $ cat b
+  This is file b2
+  $ hg merge 1 --config merge.checkunknown=ignore
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ cat b.orig
+  This is file b2
+  $ hg up --clean 2
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mv b.orig b
+
+this merge of b should work
+  $ cat b
+  This is file b2
   $ hg merge -f 1
   merging b
   merging for b