parse-options: add new function "usage_msg_opt"

parse-options: add new function "usage_msg_opt"

This function can be used instead of "usage_with_options" when you
want to print an error message before the usage string.

It may be useful because:

if (condition)
	usage_msg_opt("condition is false", usage, opts);

is shorter than:

if (condition) {
	fprintf(stderr, "condition is false\n\n");
	usage_with_options(usage, opts);
}

and may be more consistent.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>