R1:03c6bf0d0916

Make Herald less ambitious about resolving repositories for revisions

Summary:
Fixes T4636. If a user manually deletes a "repository" setting from a revision, Herald attempts to resolve it. Instead, Herald should now just trust Differential. Generally, the new logic is:

- When diffs are created, figure out repository information.
- When revisions are updated, copy info from diffs.
- Everywhere else, just trust the revision field.

Test Plan:
- Created revisions.
- Used Herald to dry-run…