Fix small bugs regarding Maniphest Task <=> Differential Revision attachments

Summary:
introduced in D3006, D3007. we need a list of phids for revision and now that its attachments its always two way.

without this patch revisions don't show up on maniphest and attaching from either mani or diffu only has the attachment show up where you did it. (since two_way = false)

Test Plan: attached revisions and tasks to one another and verified things were showing up where they should

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3011