parse-options: allow -h as a short option

Let callers provide their own handler for the short option -h even
without the flag PARSE_OPT_NO_INTERNAL_HELP, but call the internal
handler (showing usage information) if that is the only parameter.
Implement the first part by checking for -h only if parse_short_opt()
can't find it and returns -2.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Jeff King <peff@peff.net>

parse-options.h

@@ -199,7 +199,7 @@ enum {
 struct parse_opt_ctx_t {
 	const char **argv;
 	const char **out;
-	int argc, cpidx;
+	int argc, cpidx, total;
 	const char *opt;
 	int flags;
 	const char *prefix;