revlog: backout e5750c6716eb

This regresses performance of 'hg branches', presumably because it's
visiting the revlog in the wrong order. This suggests we either need
to fix the branch code or add some read-behind to mitigate the effect.

mercurial/revlog.py

@@ -812,7 +812,7 b' class revlog(object):'
         else:
             df = self.opener(self.datafile)
 
-        readahead = max(_chunksize, length)
+        readahead = max(65536, length)
         df.seek(offset)
         d = df.read(readahead)
         df.close()