test-parse-options: update to handle negative ints

Fix the printf specification to treat 'integer' as the signed type
that it is and add a test that checks that we parse negative option
arguments.

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-parse-options.c

@@ -82,7 +82,7 @@ int main(int argc, char **argv)
 	argc = parse_options(argc, (const char **)argv, prefix, options, usage, 0);
 
 	printf("boolean: %d\n", boolean);
-	printf("integer: %u\n", integer);
+	printf("integer: %d\n", integer);
 	printf("timestamp: %lu\n", timestamp);
 	printf("string: %s\n", string ? string : "(not set)");
 	printf("abbrev: %d\n", abbrev);