test-parse-options: use appropriate cast in length_callback

OPT_CALLBACK() is passed &integer which is now an "int" rather than
"unsigned long". Update the length_callback function.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-parse-options.c

@@ -15,7 +15,7 @@ int length_callback(const struct option *opt, const char *arg, int unset)
 	if (unset)
 		return 1; /* do not support unset */
 
-	*(unsigned long *)opt->value = strlen(arg);
+	*(int *)opt->value = strlen(arg);
 	return 0;
 }