R1:c429e4a22279

Don't create mentions for dependent diffs

Summary:
Fixes T6858. We shouldn't create mentions for dependent diffs.

NOTE: This won't fix the issue for existing revisions (which have the mentions edge), but I think that this is harmless.

Test Plan: Added `Depends on Dxxx` to a differential summary. Saw a `josh added a dependent revision` transaction, but no explicit mention.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest…