t-reftable-pq: make merged_iter_pqueue_check() callable by reference

t-reftable-pq: make merged_iter_pqueue_check() callable by reference

merged_iter_pqueue_check() checks the validity of a priority queue
represented by a merged_iter_pqueue struct by asserting the
parent-child relation in the struct's heap. Explicity passing a
struct to this function means a copy of the entire struct is created,
which is inefficient.

Make the function accept a pointer to the struct instead. This is
safe to do since the function doesn't modify the struct in any way.
Make the function parameter 'const' to assert immutability.

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>