dirstate: simplify dirstate's __iter__

Probably also a performance win, but not measurable in perfdirstate.

Differential Revision: https://phab.mercurial-scm.org/D269

mercurial/dirstate.py

@@ -359,8 +359,7 b' class dirstate(object):'
         return key in self._map
 
     def __iter__(self):
-        for x in sorted(self._map):
-            yield x
+        return iter(sorted(self._map))
 
     def items(self):
         return self._map.iteritems()