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>

reftable/pq.c

@@ -71,7 +71,7 @@ struct pq_entry merged_iter_pqueue_remove(struct merged_iter_pqueue *pq)
 	return e;
 }
 
-void merged_iter_pqueue_add(struct merged_iter_pqueue *pq, struct pq_entry e)
+void merged_iter_pqueue_add(struct merged_iter_pqueue *pq, const struct pq_entry *e)
 {
 	int i = 0;
 
@@ -81,7 +81,7 @@ void merged_iter_pqueue_add(struct merged_iter_pqueue *pq, struct pq_entry e)
 					    pq->cap * sizeof(struct pq_entry));
 	}
 
-	pq->heap[pq->len++] = e;
+	pq->heap[pq->len++] = *e;
 	i = pq->len - 1;
 	while (i > 0) {
 		int j = (i - 1) / 2;