commit: fix memory leak in reduce_heads()

We don't free the temporary scratch space we use with
`remove_redundant()`. Free it similar to how we do it in
`get_merge_bases_many_0()`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1080,6 +1080,7 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 	num_head = remove_redundant(array, num_head);
 	for (i = 0; i < num_head; i++)
 		tail = &commit_list_insert(array[i], tail)->next;
+	free(array);
 	return result;
 }