Function prepare_for_pull_delta 

pub(crate) async fn prepare_for_pull_delta(
    repo: &Repo,
    progress: &PullProgress,
    imgref: &ImageReference,
    delta: Arc<Delta>,
    booted_deployment: Option<&Deployment>,
) -> Result<PreparedPullResult>

Prepare an import whose metadata and layer content both come from a delta file plus the source image already in repo; nothing is fetched.