parse-options: make OPT_INTEGER's argh explicit

OPTION_INTEGER hardcodes its argh member to be "n", but the decision is
hidden deep in usage_with_options_internal(). Make "n" the default argh
for the OPT_INTEGER macro while leaving it undecided for the OPTION_INTEGER
enum.

This makes it less surprising to users that argh is "n" when using the
OPT_INTEGER macro.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Reviewed-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>