Return commit information for Revision "close" and "update" transactions over the Conduit API

Summary: Depends on D19175. Ref T13099. This fills in "close" and "update" transactions so that they show which commit(s) caused the action.

Test Plan: Used transaction.search to query some revisions, saw commit PHID information.

Maniphest Tasks: T13099

Differential Revision: https://secure.phabricator.com/D19176