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).

• 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.