dirstate: implement __len__ on dirstatemap (issue5695)

Differential Revision: https://phab.mercurial-scm.org/D884

mercurial/dirstate.py

@@ -1300,6 +1300,9 b' class dirstatemap(object):'
     def iteritems(self):
         return self._map.iteritems()
 
+    def __len__(self):
+        return len(self._map)
+
     def __iter__(self):
         return iter(self._map)