wt-status: use "format" function attribute for status_printf

These functions could benefit from the added compile-time
safety of having the compiler check printf arguments.

Unfortunately, we also sometimes pass an empty format string,
which will cause false positives with -Wformat-zero-length.
In this case, that warning is wrong because our function is
not a no-op with an empty format: it may be printing
colorized output along with a trailing newline.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.h

@@ -96,9 +96,9 @@ void wt_status_get_state(struct wt_status_state *state, int get_detached_from);
 void wt_shortstatus_print(struct wt_status *s);
 void wt_porcelain_print(struct wt_status *s);
 
-void status_printf_ln(struct wt_status *s, const char *color, const char *fmt, ...)
-	;
-void status_printf(struct wt_status *s, const char *color, const char *fmt, ...)
-	;
+__attribute__((format (printf, 3, 4)))
+void status_printf_ln(struct wt_status *s, const char *color, const char *fmt, ...);
+__attribute__((format (printf, 3, 4)))
+void status_printf(struct wt_status *s, const char *color, const char *fmt, ...);
 
 #endif /* STATUS_H */