To read a reference for the reftable stack, we first create a generic
`reftable_table` from the merged table and then read the reference via a
convenience function. We are about to remove these generic interfaces,
so let's instead open-code the logic to prepare for this removal.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>