branchmap: add a copy method

If we want branchcache of different filter to collaborate, they need a simple
way to copy each other. This will ensure that each filtered have no side effect
on other filter level cache.

mercurial/branchmap.py

@@ -117,6 +117,9 b' class branchcache(dict):'
         except IndexError:
             return False
 
+    def copy(self):
+        """return an deep copy of the branchcache object"""
+        return branchcache(self, self.tipnode, self.tiprev, self.filteredhash)
 
     def write(self, repo):
         try: