commit: use DEFINE_LIST_SORT

commit: use DEFINE_LIST_SORT

Use DEFINE_LIST_SORT to build a typed sort function for commit_list
entries instead of calling llist_mergesort().  This gets rid of the next
pointer accessor functions and their calling overhead at the cost of a
slightly increased object text size.

Before:
__TEXT	__DATA	__OBJC	others	dec	hex
18795	92	0	104654	123541	1e295	commit.o

With this patch:
__TEXT	__DATA	__OBJC	others	dec	hex
18963	92	0	106094	125149	1e8dd	commit.o

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