advice.h: add missing __attribute__((format)) & fix usage

Add the missing __attribute__((format)) checking to
advise_if_enabled(). This revealed a trivial issue introduced in
b3b18d1621 (advice: revamp advise API, 2020-03-02). We treated the
argv[1] as a format string, but did not intend to do so. Let's use
"%s" and pass argv[1] as an argument instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

advice.h

@@ -90,6 +90,7 @@ int advice_enabled(enum advice_type type);
 /**
  * Checks the visibility of the advice before printing.
  */
+__attribute__((format (printf, 2, 3)))
 void advise_if_enabled(enum advice_type type, const char *advice, ...);
 
 int error_resolve_conflict(const char *me);

t/helper/test-advise.c

@@ -16,7 +16,7 @@ int cmd__advise_if_enabled(int argc, const char **argv)
 	 * selected here and in t0018 where this command is being
 	 * executed.
 	 */
-	advise_if_enabled(ADVICE_NESTED_TAG, argv[1]);
+	advise_if_enabled(ADVICE_NESTED_TAG, "%s", argv[1]);
 
 	return 0;
 }