The "--xhtml" option is supported only in highlight < 3.0. There is no option
to enforce (X)HTML output format compatible with both highlight < 3.0 and
highlight >= 3.0. However default output format is HTML so we don't need to
explicitly specify it.
Signed-off-by: Adam Tkac <atkac@redhat.com> Helped-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>