Function export_chunk 

pub(crate) fn export_chunk<W: Write>(
    repo: &Repo,
    commit: &str,
    chunk: BTreeMap<Rc<str>, (u64, Vec<Utf8PathBuf>)>,
    out: &mut Builder<W>,
    create_parent_dirs: bool,
) -> Result<()>

Output a chunk to a tar stream.