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 Tasks: T6858
Differential Revision: https://secure.phabricator.com/D11460