t/helper: use `hash_to_hex_algop()` to print hashes

t/helper: use `hash_to_hex_algop()` to print hashes

The "reftable" test helper uses a hand-crafted version to convert from a
raw hash to its hex variant. This was done because this code used to be
part of the reftable library, where we do not use most functions from
the Git core.

Now that the code is integrated into the "dump-reftable" helper though,
that limitation went away. Let's thus use `hash_to_hex_algop()` instead.

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