R1:12cd0d0b6756

Handle symbolic commit names better in Diffusion

Summary: We now allow symbolic commits, so let them through the pipeline.

Test Plan: {F10571}

Reviewers: davidreuss, btrahan, vrana

Reviewed By: davidreuss

CC: aran

Maniphest Tasks: T1130

Differential Revision: https://secure.phabricator.com/D2315