Consolidate querying of things which we can use `git for-each-ref` for

Summary: Ref T2230. This cleans up D7442, by using `git for-each-ref` everywhere we can, in a basically reasonable way.

Test Plan:
In bare and non-bare repositories:

  - Ran discovery with `bin/repository discover`;
  - listed branches on `/diffusion/X/`;
  - listed tags on `/diffusion/X/`;
  - listed tags, branches and refs on `/diffusion/rXnnnn`.

Reviewers: btrahan, avivey

Reviewed By: avivey

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7447