The reftable code has been optimized to avoid an unnecessary
stat/reload of the stack when an addition already holds the
list_file lock, reducing the number of newfstatat syscalls from
linear to constant when writing refs.
* kn/reftable-optimize-reloading:
reftable/stack: avoid reloading the stack when already locked
reftable/stack: move list lock to `struct reftable_stack`
reftable/stack: rename reftable_stack_new_addition()
reftable/stack: remove `REFTABLE_STACK_NEW_ADDITION_RELOAD`