Allow Arcanist to parse bare revision IDs from "Differential Revisions:" field

Summary: Ref T8087. Prepares for eventually making these optional after T6030. See also T7443.

Test Plan:

• See the next change for the server-side part of this.
• With both patches applied, rigged the server to return D123.
• Created a revision, saw bare D123.
• Updated it with bare D123, things worked properly.
• Created this revision with full URIs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Maniphest Tasks: T8087

Differential Revision: https://secure.phabricator.com/D12748