Don't show "master (branched from master)" in UI

Summary:
Ref T3462. If someone works directly on master, we currently show "Branch: master (branched from master)" in the UI.

Although this is sort of technically accurate, it is confusing.

Instead, just show "Branch: master" in this situation.

Test Plan: Saw "master" instead of "master (branched from master)".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3462

Differential Revision: https://secure.phabricator.com/D14829