R1:2e0f18995059

Fix an issue with extended policy checks and `@mentions`

Summary: Ref T8463.

Test Plan:
- Created a new revision via web UI with a username `@mention` in the summary and no repository.
- Prior to patch, hit a "not attached" error.
- After patch, no error.
- Created a new web UI revision, as above, but with a repository; saw repository work fine.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8463

Differential Revision: https://secure.phabricator.com/D13205