Switch over to using a real hg repo instead of a bundle.

The 'hg' command-line tool doesn't entirely work when treating a .bundle file
as if it were a repo. This change extracts the bundle into a new hg repo
directory, and changes our test suite over to use it.

Testing done: Ran unit tests.
Add latest Diff timestamp. Review last activity does not represent diff activity.

Review last activity does not represent diff activity. One can not see if the
newer diff has been uploaded or just comment added. This change adds another
timestamp to the header of the review request box to show when the last version
of the diff was added.

Testing Done:
Tested manually on my development RB install.

Reviewed at https://reviews.reviewboard.org/r/4898/