revlog: use bytes() to ensure text from _chunks is a reasonable type

mercurial/revlog.py

@@ -1277,7 +1277,7 b' class revlog(object):'
 
         bins = self._chunks(chain, df=_df)
         if text is None:
-            text = str(bins[0])
+            text = bytes(bins[0])
             bins = bins[1:]
 
         text = mdiff.patches(text, bins)