R1:f561dc172d49

Implement a dedicated "diffusion.blame" API method

Summary:
Fixes T2451. Several motivations here, from strongest to weakest:

- Currently, getting blame and file content are closely entwined. This makes fixing T9319 more difficult, and I want to fix it. I want to separate blame from content so there's more flexibility in how we approach this issue.
- This makes pursuing T2450 easier, if it turns out to be a meaningful win.
- If we can get a win on blame performance, we can do `arc blame` eventually…