Emit "did edit task" event correctly for Maniphest comments

Summary: We emit "did edit task" properly elsewhere, but only emit "will edit task" in this case. Emit the second event correctly.

Test Plan: Added a listener, verified it got hit.

Reviewers: skrul, chad

Reviewed By: skrul

CC: aran

Differential Revision: https://secure.phabricator.com/D5419