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/merged.c

@@ -36,7 +36,7 @@ static int merged_iter_init(struct merged_iter *mi)
 				.rec = rec,
 				.index = i,
 			};
-			merged_iter_pqueue_add(&mi->pq, e);
+			merged_iter_pqueue_add(&mi->pq, &e);
 		}
 	}
 
@@ -71,7 +71,7 @@ static int merged_iter_advance_nonnull_subiter(struct merged_iter *mi,
 		return 0;
 	}
 
-	merged_iter_pqueue_add(&mi->pq, e);
+	merged_iter_pqueue_add(&mi->pq, &e);
 	return 0;
 }