reftable/stack: reorder swapping in the reloaded stack contents

reftable/stack: reorder swapping in the reloaded stack contents

The code flow of how we swap in the reloaded stack contents is somewhat
convoluted because we switch back and forth between swapping in
different parts of the stack.

Reorder the code to simplify it. We now first close and unlink the old
tables which do not get reused before we update the stack to point to
the new stack.

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