env--helper: write to opt->value in parseopt helper

We use OPT_CALLBACK_F() to call the option_parse_type() callback, passing it the address of "cmdmode" as the value to write to. But the callback doesn't look at opt->value at all, and instead writes to a global variable. This works out because that's the same global variable we happen to pass in, but it's rather confusing. Let's use the passed-in value instead. We'll also make "cmdmode" a local variable of the main function, ensuring we can't make the same mistake again. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>