R1:ecbc82da33c5

Expose "commits.add|set|remove" on "maniphest.edit" API calls

Summary: See PHI1396. Ideally this would be some kind of general-purpose tie-in to object relationships, but see D18456 for precedent.

Test Plan: Used `maniphest.edit` to edit associated commits for a task.

Differential Revision: https://secure.phabricator.com/D20731