repofilecache: directly use 'repo.vfs.join'

The 'vfs' attribute already have all methods we need, the value of going
through the repository for this is low. so we removes it.

mercurial/localrepo.py

@@ -71,7 +71,7 b' class repofilecache(scmutil.filecache):'
     """
 
     def join(self, obj, fname):
-        return obj.join(fname)
+        return obj.vfs.join(fname)
     def __get__(self, repo, type=None):
         if repo is None:
             return self