Function write_pack_until 

pub fn write_pack_until(
    plan: &PackPlan,
    dest: &Path,
    on_item: impl FnMut(usize) -> bool,
) -> Result<Option<usize>>

Like write_pack, but on_item returns whether to go on. Returning false stops after that item: nothing is left at dest (the archive is built in a temporary file that is removed) and the result is None.