R1:874a9b7fe3f8

When creating or updating a revision, infer the repository from the diff

Summary:
Ref T603. When a diff is attached to a revision, try to guess the repository if possible. In cases where we succeed, this automatically gives us intuitive policy behavior (i.e., you can see a revision if you can see the repository the change is against).

I pulled this into a funky little "Lookup" class for two reasons:

- It's used in two places;
- I anticipate that we might need to add some sort of `explainWhy()`…