commit: add a check if it is trying to close an already closed branch head

It would check if the revision we are going to close is already a
closed branch head and print the error message accordingly.

Differential Revision: https://phab.mercurial-scm.org/D6491

mercurial/commands.py

@@ -1673,7 +1673,10 b' def _docommit(ui, repo, *pats, **opts):'
     if opts.get('close_branch'):
         extra['close'] = '1'
 
-        if not bheads:
+        if repo['.'].closesbranch():
+            raise error.Abort(_('current revision is already a branch closing'
+                                ' head'))
+        elif not bheads:
             raise error.Abort(_('can only close branch heads'))
         elif branch == repo['.'].branch() and repo['.'].node() not in bheads:
             raise error.Abort(_('can only close branch heads'))

tests/test-branches.t

@@ -280,7 +280,7 b' verify update will accept invalid legacy'
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg commit -d '9 0' --close-branch -m 'close this part branch too'
   $ hg commit -d '9 0' --close-branch -m 're-closing this branch'
-  abort: can only close branch heads
+  abort: current revision is already a branch closing head
   [255]
 
   $ hg log -r tip --debug