Attach TaskPHIDs to commits in diffusion.getcommits

Summary: Uses edge query to attach TaskPHIDs to commit objects

Test Plan: Use conduit to getcommits with attached tasks

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7668