Function clone_or_fetch 

pub fn clone_or_fetch(
    url: &str,
    dest: &Path,
    ssh_key: Option<&Path>,
) -> Result<String>

Ensure dest is an up-to-date clone of url: clone if absent, else fetch. Returns the resulting HEAD SHA. SSH remotes use the deploy key at ssh_key (resolved via nornir_ssh_key_path when None).