branchcache: only iterate over branches which needs to be verified

Otherwise we loop over all the branches and call _verifybranch() even if not
required.

Differential Revision: https://phab.mercurial-scm.org/D6240

mercurial/branchmap.py

@@ -200,7 +200,8 b' class branchcache(object):'
 
     def _verifyall(self):
         """ verifies nodes of all the branches """
-        for b in self._entries:
+        needverification = set(self._entries.keys()) - self._verifiedbranches
+        for b in needverification:
             self._verifybranch(b)
 
     def __iter__(self):