If fast-export is being used to generate a fast-import stream that
will be used afterwards it is desirable to indicate the end of the
stream with the new 'done' command.
Add a flag that causes fast-export to end with 'done'.
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>