Cleanup of the tar-store code.

Cleanup of the tar-store code.

  - Make spawn_filter generic, not part of Tarfile, so that it can be used
    by other parts of the code.  (Specifically, it should be used later
    when writing out a segment descriptor, so that the descriptor can be
    filtered through gpg for signing.)
  - Drop internal_write_object.  It had only one caller, so simply inline
    the code.