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).
