R1:ff4e965c90fc

Don't try to download diffs-of-diffs

Summary:
Ref T1715. When the user clicks "Download Raw Diff" in Differential, we try to build a diff of exactly what they're seeing. However:

- This doesn't work if any of the changes have multiple hunks, and fixing it seems hard.
- I suspect this diff is never actually useful anyway? And probably kind of confusing in the best case. You can't really apply it to anyhting, since you'd have to apply another diff first.

Instead, just build the right-side diff, which…