reftable: use a pointer for pq_entry param

reftable: use a pointer for pq_entry param

The speed of the merged_iter_pqueue_add() can be improved by using a
pointer to the pq_entry struct, which is 96 bytes. Since the pq_entry
param is worked directly on the stack and does not currently have a
pointer to it, the merged_iter_pqueue_add() function is slightly
slower.

References to pq_entry in reftable have typically included pointers,
such as both of the params for pq_less().

Since we are working with pointers in the pq_entry param, as keenly
pointed out, the pq_entry param has also been made into a const since
the contents of the pq_entry param are copied and not manipulated.

Signed-off-by: Elijah Conners <business@elijahpepe.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>