branchcache: don't verify closed nodes in _branchtip()

We only do membership testing there.

Differential Revision: https://phab.mercurial-scm.org/D6210

mercurial/branchmap.py

@@ -304,7 +304,6 b' class branchcache(object):'
         otherwise return last closed head and true.'''
         tip = heads[-1]
         closed = True
-        self._verifyclosed()
         for h in reversed(heads):
             if h not in self._closednodes:
                 tip = h