Merge branch 'ps/reftable-iteration-perf'

The code to iterate over refs with the reftable backend has seen some optimization. * ps/reftable-iteration-perf: reftable/reader: add comments to `table_iter_next()` reftable/record: don't try to reallocate ref record name reftable/block: swap buffers instead of copying reftable/pq: allocation-less comparison of entry keys reftable/merged: skip comparison for records of the same subiter reftable/merged: allocation-less dropping of shadowed records reftable/record: introduce function to compare records by key