reftable/stack: refactor stack reloading to have common exit path
The `reftable_stack_reload_maybe_reuse()` function is responsible for
reloading the reftable list from disk. The function is quite hard to
follow though because it has a bunch of different exit paths, many of
which have to free the same set of resources.
Refactor the function to have a common exit path. While at it, touch up
the style of this function a bit to match our usual coding style better.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>