packfile: keep prepare_packed_git() private

The reason callers have to call this is to make sure either packed_git
or packed_git_mru pointers are initialized since we don't do that by
default. Sometimes it's hard to see this connection between where the
function is called and where packed_git pointer is used (sometimes in
separate functions).

Keep this dependency internal because now all access to packed_git and
packed_git_mru must go through get_xxx() wrappers.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -336,7 +336,6 @@ static int open_pack_bitmap(void)
 
 	assert(!bitmap_git.map && !bitmap_git.loaded);
 
-	prepare_packed_git(the_repository);
 	for (p = get_packed_git(the_repository); p; p = p->next) {
 		if (open_pack_bitmap_1(p) == 0)
 			ret = 0;