add: make warn_pathless_add() a no-op after first call

Make warn_pathless_add() print its warning the first time it is called
and do nothing if called again.  This will make it easier to show the
warning on the fly when a relevant condition is detected without
risking showing it multiple times when multiple such conditions hold.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -33,8 +33,13 @@ static const char *short_option_with_implicit_dot;
 
 static void warn_pathless_add(void)
 {
+	static int shown;
 	assert(option_with_implicit_dot && short_option_with_implicit_dot);
 
+	if (shown)
+		return;
+	shown = 1;
+
 	/*
 	 * To be consistent with "git add -p" and most Git
 	 * commands, we should default to being tree-wide, but