Make reference bit ambiguious by using just raw git object ID without object kind or size. Object kind is usually known when using it and object size is usually unknown and unimportant. NOTE: `git.oid` corresponds to atproto's `cid-link` type and not `blob` type. Using `git.oid` inside a record doesn't guarantee the existance of the object in this repository. For now, `git.oid` can be either sha1 and sha256. this might change before 1.0 launch. Signed-off-by: Seongmin Lee <git@boltless.me>