R1:dfc3506240eb

Prevent "Maniphest Tasks" field from creating empty comment when revisions are
updated

Summary:
- If you update a revision with a nonempty "Maniphest Tasks" field, an empty
comment is posted (see T586).
- The transaction email currently says "Attached revision 'Unknown
Differential Revision'", move attaching to "didWriteRevision()" to make sure the
object has been written.

Test Plan: - Attached; updated a revision.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest…