Improve low-level branch resolution in Mercurial

Summary:
Ref T7100. Ref T7108. Ref T6160. Several issues:

• High load for mercurial repositories with huge numbers of branches (T7108).
  • In Mercurial, we resolve refs individually (one hg call per ref).
  • Each repository update also updates all refs, which requires resolving all of them.
  • For repositories with a huge number of branches,
• We don't distinguish between closed branches (a Mercurial-only concept) and open branches (T6160).
  • In Git, when a branch is merged, it ceases to exist.
  • In Mercurial, when a branch is merged, it still exists, it's just "closed". Normally, no one cares about these branches.
  • In the low-level query, correctly identify which refs we resolve as branches.
  • In the low-level query, correctly mark closed branches as closed.
• This marginally improves ref handling in general (see T7100).

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6160, T7108, T7100

Differential Revision: https://secure.phabricator.com/D12548