Return Diffusion diffs through Files, not directly over Conduit

Summary:
Fixes T10423. Ref T11524. This changes diffusion.rawdiffquery to return a file PHID instead of a blob of data.

This is better in general, but particularly better for huge diffs (as in T10423) and diffs with non-utf8 data (as in T10423).

Test Plan:

• Used bin/differential extract to extract a latin1 diff, got a clean diff.
• Used bin/repository reparse --herald to rerun herald on a latin1 diff, got a clean result.
• Pushed latin1 diffs to test commit hooks.
• Triggered the the too large / too slow logic.
• Viewed latin1 diffs in Diffusion.
• Used "blame past this change" in Diffusion to hit the before logic.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T10423, T11524

Differential Revision: https://secure.phabricator.com/D16460