Convert lookup_replace_object to struct object_id

Convert lookup_replace_object to struct object_id

Convert both the argument and the return value to be pointers to struct
object_id.  Update the callers and their internals to deal with the new
type.  Remove several temporaries which are no longer needed.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>