revlog: use iterbytestr()

Otherwise we iterate over integers in Python 3 and the
character compare fails.

Differential Revision: https://phab.mercurial-scm.org/D5947

mercurial/revlog.py

@@ -1340,7 +1340,7 b' class revlog(object):'
             return True
 
         def maybewdir(prefix):
-            return all(c == 'f' for c in prefix)
+            return all(c == 'f' for c in pycompat.iterbytestr(prefix))
 
         hexnode = hex(node)