revlog: use bytes() instead of str() to get data from memoryview

Fixes `files -v` on Python 3.

mercurial/revlog.py

@@ -1234,7 +1234,7 b' class revlog(object):'
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions"""
         if rev1 != nullrev and self.deltaparent(rev2) == rev1:
-            return str(self._chunk(rev2))
+            return bytes(self._chunk(rev2))
 
         return mdiff.textdiff(self.revision(rev1),
                               self.revision(rev2))