Introduce advise() to print hints

Like error(), warn(), and die(), advise() prints a short message
with a formulaic prefix to stderr.

It is local to revert.c for now because I am not sure this is
the right API (we may want to take an array of advice lines or a
boolean argument for easy suppression of unwanted advice).

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

builtin/revert.c

@@ -241,6 +241,15 @@ static void set_author_ident_env(const char *message)
 			sha1_to_hex(commit->object.sha1));
 }
 
+static void advise(const char *advice, ...)
+{
+	va_list params;
+
+	va_start(params, advice);
+	vreportf("hint: ", advice, params);
+	va_end(params);
+}
+
 static char *help_msg(void)
 {
 	struct strbuf helpbuf = STRBUF_INIT;