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 should align well with user expectation and doesn't suffer from the multi-hunk bug.

Some day, we could maybe add some of the fancy options in T1715.

See: https://github.com/facebook/phabricator/issues/461

Test Plan: Downloaded a multi-hunk diff, got the original back and applied it cleanly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1715

Differential Revision: https://secure.phabricator.com/D7694