Merge branch 'ps/reftable-iteration-perf-part2' into ps/reftable-reflog-iteration-perf

* ps/reftable-iteration-perf-part2: refs/reftable: precompute prefix length reftable: allow inlining of a few functions reftable/record: decode keys in place reftable/record: reuse refname when copying reftable/record: reuse refname when decoding reftable/merged: avoid duplicate pqueue emptiness check reftable/merged: circumvent pqueue with single subiter reftable/merged: handle subiter cleanup on close only reftable/merged: remove unnecessary null check for subiters reftable/merged: make subiters own their records reftable/merged: advance subiter on subsequent iteration reftable/merged: make `merged_iter` structure private reftable/pq: use `size_t` to track iterator index