Summary: add a conduit method to enable querying revisions' phid from
their revision_IDs, and another one to update the task->revision assoc.
Test Plan: for querying revision_phid method, tested empty, one, and two
revisions in the query. For the one to update the task->revision assoc,
I have another diff in facebook which verified it add and remove assoc
correctly.
Reviewers: tuomaspelkonen, epriestley
CC:
Differential Revision: 165