radicle: Compute root OID for older remotes — heartwood

For older clients, refs/rad/root won’t be set. In that case, compute it locally based on refs/rad/id.

     }
     fn identity_root_of(&self, remote: &RemoteId) -> Result<Oid, RepositoryError> {
         self.reference_oid(remote, &git::refs::storage::IDENTITY_ROOT)
             .map_err(RepositoryError::from)
         // Remotes that run newer clients will have this reference set. For older clients,
         // compute the root OID based on the identity head.
         if let Ok(root) = self.reference_oid(remote, &git::refs::storage::IDENTITY_ROOT) {
             return Ok(root);
         }
         let oid = self.identity_head_of(remote)?;
         let root = self
             .revwalk(oid)?
             .last()
             .ok_or(RepositoryError::Doc(DocError::Missing))??;
         Ok(root.into())
     }
     fn canonical_identity_head(&self) -> Result<Oid, RepositoryError> {