t-reftable-stack: use reftable_ref_record_equal() to compare ref records

t-reftable-stack: use reftable_ref_record_equal() to compare ref records

In the current stack tests, ref records are compared for equality
by sometimes using the dedicated function for ref-record comparison,
reftable_ref_record_equal(), and sometimes by explicity comparing
contents of the ref records.

The latter method is undesired because there can exist unequal ref
records with some of the contents being equal. Replace the latter
instances of ref-record comparison with the former. This has the
added benefit of preserving uniformity throughout the test file.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>