branchcache: don't verify while creating a copy

The copy will not be mark as verified, so there is no need to verify nodes.
Thanks to Yuya who spotted this while reviewing.

Differential Revision: https://phab.mercurial-scm.org/D6238

mercurial/branchmap.py

@@ -339,7 +339,6 b' class branchcache(object):'
 
     def copy(self):
         """return an deep copy of the branchcache object"""
-        self._verifyall()
         return type(self)(
             self._entries, self.tipnode, self.tiprev, self.filteredhash,
             self._closednodes)