Function update_detached_metadata 

pub fn update_detached_metadata<D: Write, C: IsA<Cancellable>>(
    src: impl Read,
    dest: D,
    detached_buf: Option<&[u8]>,
    cancellable: Option<&C>,
) -> Result<D>

Replace the detached metadata in an tar stream which is an export of an OSTree commit.