t-reftable-merged: use reftable_ref_record_equal to compare ref records

t-reftable-merged: use reftable_ref_record_equal to compare ref records

In the test t_merged_single_record() defined in t-reftable-merged.c,
the 'input' and 'expected' ref records are checked for equality
by comparing their update indices. It is very much possible for
two different ref records to have the same update indices. Use
reftable_ref_record_equal() instead for a stronger check.

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>