test-mergesort: use DEFINE_LIST_SORT

test-mergesort: use DEFINE_LIST_SORT

Build a typed sort function for the mergesort performance test tool
using DEFINE_LIST_SORT instead of calling llist_mergesort().  This gets
rid of the next pointer accessor functions and improves the performance
at the cost of a slightly higher object text size.

Before:
0071.12: llist_mergesort() unsorted    0.24(0.22+0.01)
0071.14: llist_mergesort() sorted      0.12(0.10+0.01)
0071.16: llist_mergesort() reversed    0.12(0.10+0.01)

__TEXT	__DATA	__OBJC	others	dec	hex
6407	276	0	24701	31384	7a98	t/helper/test-mergesort.o

With this patch:
0071.12: DEFINE_LIST_SORT unsorted     0.22(0.21+0.01)
0071.14: DEFINE_LIST_SORT sorted       0.11(0.10+0.01)
0071.16: DEFINE_LIST_SORT reversed     0.11(0.10+0.01)

__TEXT	__DATA	__OBJC	others	dec	hex
6615	276	0	25832	32723	7fd3	t/helper/test-mergesort.o

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>