branchmap: prevent using __getitem__() in branchheads()

branchheads() can directly use self._entries instead.

Differential Revision: https://phab.mercurial-scm.org/D6205

mercurial/branchmap.py

@@ -290,7 +290,7 b' class branchcache(object):'
         return (n for n in nodes if n not in self._closednodes)
 
     def branchheads(self, branch, closed=False):
-        heads = self[branch]
+        heads = self._entries[branch]
         if not closed:
             heads = list(self.iteropen(heads))
         return heads