t-reftable-merged: improve the const-correctness of helper functions

t-reftable-merged: improve the const-correctness of helper functions

In t-reftable-merged.c, a number of helper functions used by the
tests can be re-defined with parameters made 'const' which makes
it easier to understand if they're read-only or not. Re-define
these functions along these lines.

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>