This change makes a few improvements to our Mercurial support:

1. The way that binary files were appearing in diffs was causing the
   diff parser (based on the GitDiffParser) to override the commit IDs
   in the filediff revisions with the file SHAs that `hg diff --git` was
   adding. The file SHAs have no meaning to us, so these have been
   replaced back with the commit IDs.
2. I've started seeing warnings pop up about unclosed resources when
   dealing with Popen. Treating the object as a context manager fixes
   this.

Testing Done:
- Ran unit tests.
- Posted a bunch of changes, including binary files.

Reviewed at https://reviews.reviewboard.org/r/13602/

asfa