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()` method if users find the heuristics confusing.

Test Plan: Created and updated revisions, saw them pick up the correct repository association. Ran Herald dry run against associable and nonassociable revisions, saw correct values populate.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7147