builtin/rev-parse: convert to struct object_id

builtin/rev-parse: convert to struct object_id

Some of the functions converted are callers of lookup_commit_reference.
However, the changes involved in converting the entire thing are not too
large, so we might as well convert it all.

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