Reference task based on branch name in `arc diff`

Summary:
This was mentioned in T2928 and nobody objected.
It just references the task instead of fixing it as that would be too aggressive.
It also doesn't check assignee of the task (by purpose).

Test Plan: Created diff from a branch named T2928.

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2928

Differential Revision: https://secure.phabricator.com/D5640