add -i (built-in): send error messages to stderr

The Perl version of that command already does that since a301973641
(add -p: print errors in separate color, 2009-02-05). The built-in
version's development started by reimplementing the initial version from
5cde71d64a (git-add --interactive, 2006-12-10) for simplicity, though,
which still printed error messages to stdout.

Let's fix that by imitating the Perl version's behavior in the built-in
version of that command.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.c

@@ -365,7 +365,7 @@ static ssize_t list_and_choose(struct add_i_state *s,
 
 			if (from < 0 || from >= items->items.nr ||
 			    (singleton && from + 1 != to)) {
-				color_fprintf_ln(stdout, s->error_color,
+				color_fprintf_ln(stderr, s->error_color,
 						 _("Huh (%s)?"), p);
 				break;
 			} else if (singleton) {