Fix an issue with arc branch and very old branches

Summary:
In D9595, we stopped parsing short-form "Differential Revision:" fields in commit messages, and only accept URLs.

I have one of the older style commit messages in my local arcanist/, so now we go down this parse failure branch in arc branch. This has never worked quite correctly, and if the parse fails we end up with a bad branch dictionary that is missing fields.

Test Plan: Ran arc branch in a working copy with an old Differential Revision: field at the head of a branch. Before patch: explosions; after patch: works great.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9620