Checking just for the unicode data files' existence is not sufficient;
we should also download them if a newer version exists on the Unicode
consortium's servers. Option -N of wget does this nicely for us.
Reviewed-by: Torsten Bögershausen <tboegi@web.de> Signed-off-by: Beat Bolli <dev+git@drbeat.li> Signed-off-by: Junio C Hamano <gitster@pobox.com>