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