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 Tasks: T685

Differential Revision: 1223