Accept pushes with arbitrary Git refs

Summary:
Depends on D20419. Ref T13277. Fixes T8936. Fixes T9383. Fixes T12300. When you push arbitrary refs to Phabricator, the push log currently complains if those refs are not tags or branches.

Upstream Git now features "notes", and there's no reason to prevent writes to arbitrary refs, particularly beause we plan to start using them soon (see T13278).

Allow these writes as affecting raw refs.

Test Plan:

• Pushed an arbitrary ref.
• Pushed some Git notes.
• Wrote a Herald ref rule, saw "ref" in the dropdown.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277, T8936, T9383, T12300

Differential Revision: https://secure.phabricator.com/D20420