localrepo: use str for lookup in vars()

vars() returns a dict of str. So always use a native str for
the key lookup.

Differential Revision: https://phab.mercurial-scm.org/D7227

mercurial/localrepo.py

@@ -1417,7 +1417,7 b' class localrepository(object):'
 
     def _refreshchangelog(self):
         """make sure the in memory changelog match the on-disk one"""
-        if b'changelog' in vars(self) and self.currenttransaction() is None:
+        if 'changelog' in vars(self) and self.currenttransaction() is None:
             del self.changelog
 
     @property