Code clean-ups.
* ps/reftable-libgit2-cleanup:
refs/reftable: always reload stacks when creating lock
reftable: don't second-guess errors from flock interface
reftable/stack: handle outdated stacks when compacting
reftable/stack: allow passing flags to `reftable_stack_add()`
reftable/stack: fix compiler warning due to missing braces
reftable/stack: reorder code to avoid forward declarations
reftable/writer: drop Git-specific `QSORT()` macro
reftable/writer: fix type used for number of records
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