Add a PHID to Changesets
Ref T13605. Changesets currently have no PHID, which limits their ability to use standard API infrastructure.
Give them a PHID, since there's no reason they don't have one other than their age.
- Ran migrations, saw PHIDs populated.
- Created new changesets, saw PHIDs.
Maniphest Tasks: T13605
Differential Revision: https://secure.phabricator.com/D21557