reftable/stack: use lock_file when adding table to "tables.list"

reftable/stack: use lock_file when adding table to "tables.list"

When modifying "tables.list", we need to lock the list before updating
it to ensure that no concurrent writers modify the list at the same
point in time. While we do this via the `lock_file` subsystem when
compacting the stack, we manually handle the lock when adding a new
table to it. While not wrong, it is at least inconsistent.

Refactor the code to consistently lock "tables.list" via the `lock_file`
subsytem.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>