fetch-pack: use DEFINE_LIST_SORT

fetch-pack: use DEFINE_LIST_SORT

Build a static typed ref sorting function using DEFINE_LIST_SORT along
with a typed comparison function near its only two callers instead of
having an exported version that calls 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
23231	389	0	113689	137309	2185d	fetch-pack.o
29158	80	0	146864	176102	2afe6	remote.o

With this patch:
__TEXT	__DATA	__OBJC	others	dec	hex
23591	389	0	117759	141739	229ab	fetch-pack.o
29070	80	0	145718	174868	2ab14	remote.o

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