Panic when using dir_push_all

[rbran]

Because the function FilesystemWriter::dir_push_all don't insert the uid/gid to the lookup table, this unwrap will create a panic because it will be unable to find the uid/gid after a FilesystemWriter::write is called.

PoC

fn test() {
    let mut writer = std::io::Cursor::new(vec![]);
    let mut fs = FilesystemWriter::default();
    let header = NodeHeader { permissions: 0, uid: 1, gid: 2, mtime: 3 };
    fs.push_dir_all("a/b/c/d/e/f/g", header).unwrap();
    fs.write(&mut writer).unwrap();
}