Merge branch 'ps/reftable-write-optim'

Code to write out reftable has seen some optimization and simplification. * ps/reftable-write-optim: reftable/block: reuse compressed array reftable/block: reuse zstream when writing log blocks reftable/writer: reset `last_key` instead of releasing it reftable/writer: unify releasing memory reftable/writer: refactorings for `writer_flush_nonempty_block()` reftable/writer: refactorings for `writer_add_record()` refs/reftable: don't recompute committer ident reftable: remove name checks refs/reftable: skip duplicate name checks refs/reftable: perform explicit D/F check when writing symrefs refs/reftable: fix D/F conflict error message on ref copy