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>

fast-import.c

@@ -3473,7 +3473,6 @@ int cmd_main(int argc, const char **argv)
 		rc_free[i].next = &rc_free[i + 1];
 	rc_free[cmd_save - 1].next = NULL;
 
-	prepare_packed_git(the_repository);
 	start_packfile();
 	set_die_routine(die_nicely);
 	set_checkpoint_signal();