[PATCH 3 of 3] revlog: read index data using mmap

[timeless]

http://bugs.python.org/issue16212

Title: mmap() dumps core upon resizing the underlying file
Type: crash
Stage: needs patch
Components: IO
Versions: Python 3.4, Python 3.2, Python 3.3

> It's not specific to the zero size truncation, it's enough if the file size decreases beyond a page border.

The issue has been stalled for about 4 years. I don't expect it to go anywhere.