commit: add ui.allowemptycommit config option

This adds a config flag that enables a user to make empty commits.
This is useful in a number of cases.

For instance, automation that creates release branches via
bookmarks may want to make empty commits to that release bookmark so that it
can't be fast-forwarded and so it can record information about the release
bookmark's creation. This is already possible with named branches, so making it
possible for bookmarks makes sense.

Another case we've wanted it is for mirroring repositories into Mercurial. We
have automation that syncs commits into hg by running things from the command
line. The ability to produce empty commits is useful for syncing unusual commits
from other VCS's.

In general, allowing the user to create the DAG as they see fit seems useful,
and when I mentioned this in IRC more than one person piped up and said they
were already hacking around this limitation by using mq, import, and
commit-dummy-change-then-amend-the-content-away style solutions.

mercurial/localrepo.py

@@ -1463,7 +1463,8 b' class localrepository(object):'
                                             text, user, date, extra)
 
             allowemptycommit = (wctx.branch() != wctx.p1().branch() or force
-                                or extra.get('close') or merge or cctx.files())
+                                or extra.get('close') or merge or cctx.files()
+                                or self.ui.configbool('ui', 'allowemptycommit'))
             if not allowemptycommit:
                 return None
 

tests/test-commit.t

@@ -544,6 +544,18 b' commit copy'
        0         0       6  .....       0 26d3ca0dfd18 000000000000 000000000000 (re)
        1         6       7  .....       1 d267bddd54f7 26d3ca0dfd18 000000000000 (re)
 
+Test making empty commits
+  $ hg commit --config ui.allowemptycommit=True -m "empty commit"
+  $ hg log -r . -v --stat
+  changeset:   2:d809f3644287
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  description:
+  empty commit
+  
+  
+  
 verify pathauditor blocks evil filepaths
   $ cat > evil-commit.py <<EOF
   > from mercurial import ui, hg, context, node
@@ -568,7 +580,7 b' verify pathauditor blocks evil filepaths'
 #endif
 
   $ hg rollback -f
-  repository tip rolled back to revision 1 (undo commit)
+  repository tip rolled back to revision 2 (undo commit)
   $ cat > evil-commit.py <<EOF
   > from mercurial import ui, hg, context, node
   > notrc = "HG~1/hgrc"
@@ -586,7 +598,7 b' verify pathauditor blocks evil filepaths'
   [255]
 
   $ hg rollback -f
-  repository tip rolled back to revision 1 (undo commit)
+  repository tip rolled back to revision 2 (undo commit)
   $ cat > evil-commit.py <<EOF
   > from mercurial import ui, hg, context, node
   > notrc = "HG8B6C~2/hgrc"