Expose transaction PHID in maniphest.gettasktransactions

Summary: Fixes T6629.

Test Plan: {F238271}

Reviewers: btrahan, chad, 20after4

Reviewed By: 20after4

Subscribers: 20after4, epriestley

Maniphest Tasks: T6629

Differential Revision: https://secure.phabricator.com/D10898