py3: use string for "close" value in commit extras

The extras dict on commits has string keys and values. Before this
commit, we passed "close" as an integer, which Python 2 gladly
coerced to a string during a %s formatting in the bowels of
changelog.py.

Differential Revision: https://phab.mercurial-scm.org/D2157

mercurial/commands.py

@@ -1551,7 +1551,7 b' def _docommit(ui, repo, *pats, **opts):'
 
     extra = {}
     if opts.get('close_branch'):
-        extra['close'] = 1
+        extra['close'] = '1'
 
         if not bheads:
             raise error.Abort(_('can only close branch heads'))