Convert all uses of unsigned char [20] to struct object_id. Switch one
use of get_sha1_hex to parse_oid_hex to avoid the need for a constant.
This change is necessary in order to convert parse_object.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>