R1:9728c65e9358

Drive blame generation through `diffusion.blame`

Summary:
Ref T2450. Ref T9319. This is still a bit messy, but not quite so bad as it was: instead of using a single call to get both blame information and file content, use `diffusion.blame` for blame information.

This will make optimizations to both blame and file content easier.

Test Plan: Viewed a bunch of blame (color on/off, blame on/off).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2450, T9319

Differential Revision: https://secure.