git: implement stub prefetch_parents dirstate method

A recent change (35b255e474d9) introduced this new required dirstate method
but didn't update the git extension.

hgext/git/dirstate.py

@@ -288,6 +288,10 b' class gitdirstate(object):'
         # TODO: track copies?
         return None
 
+    def prefetch_parents(self):
+        # TODO
+        pass
+
     @contextlib.contextmanager
     def parentchange(self):
         # TODO: track this maybe?