revlog: backout e5750c6716eb stable

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.

--- a/mercurial/revlog.py	Thu Apr 26 03:47:17 2012 +0200
+++ b/mercurial/revlog.py	Fri Apr 27 13:07:29 2012 -0500
@@ -812,7 +812,7 @@
         else:
             df = self.opener(self.datafile)
 
-        readahead = max(_chunksize, length)
+        readahead = max(65536, length)
         df.seek(offset)
         d = df.read(readahead)
         df.close()