Added support for displaying interdiffs (diffs between revisions of diffs). This allows users to see what's changed between two versions of a diff.

This is not complete. Comments don't yet work (I'll be working on a change to how comments are stored which will allow them to work in interdiffs later). There's also some things that could use some cleaning up (faster logic in one spot, possibly hide unchanged files in the diff view). Due to this, I didn't expose any UI. It's still useful to look at, though.

Uploaded two revisions of a diff. The interdiff looked correct.