Be able to select both versions of diff to compare to

The itch that I wanted to scratch was that it was impossible when looking at diff 3, to look between the diffs 1 and 2 without needing a double refresh (or some intimate URL knowledge).

Didn't want to change too much at the start, since I was unfamiliar with the project. Based on the feedback I got from the review I ended up changing a little bit more: I merged the "Jump to revision" and the "Changes between rX and" buttons.

Looks a lot cleaner. Anyway, check the review for some screenshots:

http://reviews.reviewboard.org/r/3477/diff/#index_header