mergestate: make unresolved() use iteritems()

mergestate.unresolved() is a generator, so it seems better for it to
rely on iteritems() than items(), although it also seems unlikely for
it to make a noticeable difference.

mercurial/merge.py

@@ -444,7 +444,7 b' class mergestate(object):'
     def unresolved(self):
         """Obtain the paths of unresolved files."""
 
-        for f, entry in self._state.items():
+        for f, entry in self._state.iteritems():
             if entry[0] == 'u':
                 yield f