Ability to diff between any two *commits* (not revisions of a single file) (BB-6045)

In the list of commits (URL: /<username>/<projectname>/changesets), let’s have some UI to select any two commits and see a diff between them.

A possible UI might look like this: every commit has a little icon to its left which symbolises “diff”. When you click on one, it just gets depressed. Perhaps a message could show somewhere to invite you to click on another one. Once you click on a second one, the page reloads and a diff between the two commits is shown.