Improve git parent branch detection in post-review.

This adds improved logic for determining the parent branch used when generating
a diff. It will attempt to detect the merge base of the current head, falling
back to defaults if necessary. A tracking branch can be manually specified
too.

The new code attempts the following:

1) Tries to detect the tracking branch for the current HEAD, if possible.
2) If a tracking branch doesn't exist, or is not remote, it will fall back
   on 'origin/master'.
3) If the --tracking-branch parameter is specified, this branch will be used
   instead of 'origin/master'.

It also provides unit tests for post-review. Right now, these test only
GitClient (in pure Git mode), but new tests can be added down the line for
the rest of post-review.

Patch by Dan Savilonis
Reviewed at http://reviews.reviewboard.org/r/1144/

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