push: use "repo['.']" instead of old form "repo['']"

Note that this does not conflict with the commit message of my
previous patch: I found this after writing the previous patch
(besides, I very easily forget things).

Differential Revision: https://phab.mercurial-scm.org/D2965

mercurial/commands.py

@@ -4166,7 +4166,7 b' def push(ui, repo, dest=None, **opts):'
     repo._subtoppath = dest
     try:
         # push subrepos depth-first for coherent ordering
-        c = repo['']
+        c = repo['.']
         subs = c.substate # only repos that are committed
         for s in sorted(subs):
             result = c.sub(s).push(opts)

mercurial/subrepo.py

@@ -724,7 +724,7 b' class hgsubrepo(abstractsubrepo):'
         ssh = opts.get('ssh')
 
         # push subrepos depth-first for coherent ordering
-        c = self._repo['']
+        c = self._repo['.']
         subs = c.substate # only repos that are committed
         for s in sorted(subs):
             if c.sub(s).push(opts) == 0: