R1:c55de86f0ef0

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.