This change adds support for interdiffs, both in the backend and in the web UI.

While we had some basic hidden support for interdiffs before, it was very buggy. It only really worked when the set of files were the same. Now we handle the case where the set of files between the two revisions change.

We also allow commenting on interdiffs. The comments will show up on the review and it will indicate the revision range. This prompted another change that's been a long time coming, which is to prevent the problem where a person is commenting on a diff and the author then uploads a new diff, causing the comments to "disappear."

We now have a new diff header above the file index that shows the current revision, a list of other revisions to jump to, and a little interdiff selector. If the user has comments on a diff other than the displayed one, we display an info box telling them and showing which revisions (and interdiffs) have comments. The user can then delete them if they choose.

Reviews are no longer tied to a specific revision of a diff. A user can have comments on older diffs in the review. It's now their responsibility to make sure they're still valid. Still better than us just "losing" them.

Uploaded several iterations of a diff and left comments on various revisions, as well as on some interdiffs. Saw the info box appear. It showed me each revision with comments. I tested the cases where the comments are only on normal diffs you're not seeing, on diffs you are seeing, on interdiffs you're not seeing, and on interdiffs you are seeing.  It worked fine in each case.

Left comments on the various revisions and verified I was seeing every one of them in the review. Published the review and saw each one appear on the page.

Created a diff that modified three files and uploaded it. Then I created another that had a new file not found in the first diff, and reverted a file that was in the first diff. The resulting interdiff correctly showed new changes for the new file and deletes for the changes in the old file (as well as saying "Reverted" in the revision area).