test-mergesort: use DEFINE_LIST_SORT_DEBUG

test-mergesort: use DEFINE_LIST_SORT_DEBUG

Define a typed sort function using DEFINE_LIST_SORT_DEBUG for the
mergesort sanity check instead of using llist_mergesort().  This gets
rid of the next pointer accessor functions and improves the performance
at the cost of slightly bigger object text.

Before:
Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):     108.4 ms ±   0.2 ms    [User: 106.7 ms, System: 1.2 ms]
  Range (min … max):   108.0 ms … 108.8 ms    27 runs

__TEXT	__DATA	__OBJC	others	dec	hex
6251	276	0	23172	29699	7403	t/helper/test-mergesort.o

With this patch:
Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):      94.0 ms ±   0.2 ms    [User: 92.4 ms, System: 1.1 ms]
  Range (min … max):    93.7 ms …  94.5 ms    31 runs

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

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