fsck: convert static functions to struct object_id

Convert two static functions to use struct object_id and parse_oid_hex,
instead of relying on harcoded 20 and 40-based constants.

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