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