refs/reftable: figure out hash via `reftable_stack`

refs/reftable: figure out hash via `reftable_stack`

The function `read_ref_without_reload()` accepts a ref store as input
only so that we can figure out the hash function used by it. This is
duplicate information though because the reftable stack knows about its
hash function, too.

Drop the superfluous parameter to simplify the calling convention a bit.

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