dirstate.invalidate: avoid rebuilding _map

Since hasattr will call __getattr__, the call to hasattr(self, '_dirs')
will end up reparsing the dirstate file.

mercurial/dirstate.py

@@ -164,8 +164,8 b' class dirstate(object):'
 
     def invalidate(self):
         for a in "_map _copymap _branch _pl _dirs _ignore".split():
-            if hasattr(self, a):
-                self.__delattr__(a)
+            if a in self.__dict__:
+                delattr(self, a)
         self._dirty = 0
 
     def copy(self, source, dest):