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>

t/t1502-rev-parse-parseopt.sh

@@ -40,7 +40,7 @@ extra1    line above used to cause a segfault but no longer does
 EOF
 
 test_expect_success 'test --parseopt help output' '
-	git rev-parse --parseopt -- -h > output < optionspec
+	test_expect_code 129 git rev-parse --parseopt -- -h > output < optionspec &&
 	test_cmp expect output
 '