[PATCH] fetch.c: Remove some duplicated code in process()

It does not matter if we call prefetch() or set the TO_SCAN flag before
or after adding the object to process_queue.  However, doing it before
object_list_insert() allows us to kill 3 lines of duplicated code.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Junio C Hamano <junkio@cox.net>