t1502 (rev-parse --parseopt): test exit code from "-h"

t1502 (rev-parse --parseopt): test exit code from "-h"

rev-parse --parseopt exits with code 129 (usage error) when asked
to dump usage with -h on behalf of another command.  Scripts can
take advantage of this to avoid trying to parse usage information
as though it were the regular output from some git command.

Noticed with an &&-chaining tester.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>