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>

remote.h

@@ -207,9 +207,7 @@ struct ref *find_ref_by_name(const struct ref *list, const char *name);
 struct ref *alloc_ref(const char *name);
 struct ref *copy_ref(const struct ref *ref);
 struct ref *copy_ref_list(const struct ref *ref);
-void sort_ref_list(struct ref **, int (*cmp)(const void *, const void *));
 int count_refspec_match(const char *, struct ref *refs, struct ref **matched_ref);
-int ref_compare_name(const void *, const void *);
 
 int check_ref_type(const struct ref *ref, int flags);