branchmap: _updatebranchmap does not need to be filtered

The `_updatebranchmap` method on repo does not need to be filtered as all
callers are already handling filtering themself.

The fact it is filtered may had even lead to buggy behaviors, but by chances the method
make very sparse use of the repo object.

mercurial/localrepo.py

@@ -730,7 +730,6 b' class localrepository(object):'
             bt[bn] = self._branchtip(heads)
         return bt
 
-    @unfilteredmethod # Until we get a smarter cache management
     def _updatebranchcache(self, partial, ctxgen):
         """Given a branchhead cache, partial, that may have extra nodes or be
         missing heads, and a generator of nodes that are at least a superset of