sha1_file: convert cached object code to struct object_id

sha1_file: convert cached object code to struct object_id

Convert the code that looks up cached objects to use struct object_id.
Adjust the lookup for empty trees to use the_hash_algo.  Note that we
don't need to be concerned about the hard-coded object ID in the
empty_tree object since we never use it.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>