branchmap: dynamically resolve type of branchcache class

This is required to support subclassing.
Thanks to Yuya for suggesting this in D6151.

Differential Revision: https://phab.mercurial-scm.org/D6204

mercurial/branchmap.py

@@ -305,7 +305,7 b' class branchcache(object):'
 
     def copy(self):
         """return an deep copy of the branchcache object"""
-        return branchcache(
+        return type(self)(
             self._entries, self.tipnode, self.tiprev, self.filteredhash,
             self._closednodes)