t-reftable-merged: improve the test t_merged_single_record()

t-reftable-merged: improve the test t_merged_single_record()

In t-reftable-merged.c, the test t_merged_single_record() ensures
that a ref ('a') which occurs in only one of the records ('r2')
can be retrieved. Improve this test by adding another record 'r3'
to ensure that ref 'a' only occurs in 'r2' and that merged tables
don't simply read the last record.

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>