subrepo: use repo['.'] instead of repo['']

The "state" value (a revision) passed to abstractsubrepo.phase() can
be '' to represent the currently checked out revisions. Let's convert
that to the more common '.'.

I think this is the last of use of repo['.'] in core.

Differential Revision: https://phab.mercurial-scm.org/D3019

mercurial/subrepo.py

@@ -603,7 +603,7 b' class hgsubrepo(abstractsubrepo):'
 
     @annotatesubrepoerror
     def phase(self, state):
-        return self._repo[state].phase()
+        return self._repo[state or '.'].phase()
 
     @annotatesubrepoerror
     def remove(self):