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

src/applications/differential/storage/DifferentialRevision.php

@@ -65,6 +65,7 @@ final class DifferentialRevision extends DifferentialDAO
       ->setViewPolicy($view_policy)
       ->setAuthorPHID($actor->getPHID())
       ->attachRelationships(array())
+      ->attachRepository(null)
       ->setStatus(ArcanistDifferentialRevisionStatus::NEEDS_REVIEW);
   }