commit: show active bookmark in commit editor helper text

If there is an active bookmark while committing, the bookmark name
will be visible inside the commit message helper, below the branch
name.

This should make easier for the user to detect a mistaken commit
parent, while working for example with a bookmark centric workflow
like topic branches.

The active bookmark is checked to be in the working directory, as
pointed by Kevin Bullock, because otherwise committing would not
advance it. In other words, this would not show the active
bookmark name if the user changed the working tree parents with
'hg debugsetparents', for example.

mercurial/cmdutil.py

@@ -12,6 +12,7 b' import util, scmutil, templater, patch, '
 import match as matchmod
 import subrepo, context, repair, graphmod, revset, phases, obsolete
 import changelog
+import bookmarks
 import lock as lockmod
 
 def parsealiases(cmd):
@@ -1793,6 +1794,8 b' def commitforceeditor(repo, ctx, subs):'
         edittext.append(_("HG: branch merge"))
     if ctx.branch():
         edittext.append(_("HG: branch '%s'") % ctx.branch())
+    if bookmarks.iscurrent(repo):
+        edittext.append(_("HG: bookmark '%s'") % repo._bookmarkcurrent)
     edittext.extend([_("HG: subrepo %s") % s for s in subs])
     edittext.extend([_("HG: added %s") % f for f in added])
     edittext.extend([_("HG: changed %s") % f for f in modified])

tests/test-commit.t

@@ -263,6 +263,7 b' test commit message content'
   $ cd commitmsg
   $ echo changed > changed
   $ echo removed > removed
+  $ hg book currentbookmark
   $ hg ci -qAm init
 
   $ hg rm removed
@@ -277,6 +278,7 b' test commit message content'
   HG: --
   HG: user: test
   HG: branch 'default'
+  HG: bookmark 'currentbookmark'
   HG: added added
   HG: changed changed
   HG: removed removed