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