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 if we want.