Convert the various open_istream variants to take a pointer to struct
object_id. Introduce a temporary, which will be removed later, to work
around the fact that lookup_replace_object still returns a pointer to
unsigned char.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>