When adding {Fnnn} file references by editing comments, link objects to files

Summary: Fixes T6037. We don't currently write the "this file is attached to such-and-such object" edge on comment edits.

Test Plan: Edited a comment, adding {Fnnn}. Verified file was not attached before the edit, but was afterward.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6037

Differential Revision: https://secure.phabricator.com/D10423