Improve hg arc diff perf when image files are uploaded

Summary:
arc diff called 'hg cat' twice for every image binary in the diff.
This turns out to take 1 second per call on a large repo because mercurial
has to parse the manifest every time.

Now arc diff batches up all the files and does only two 'hg cat'
commands.  This makes the cost constant relative to the number of
images being uploaded.

Test Plan:
Ran arc diff on a diff with 30 images on both git and hg.
Verified that it was fast and that the images showed up in the web ui.

Reviewers: epriestley

Reviewed By: epriestley

CC: sid0, dschleimer, bos, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5144