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 revisions before and after a manual edit to remove the repository setting.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4636

Differential Revision: https://secure.phabricator.com/D8576