wt-status: fix 'fprintf' compilation warning

color_fprintf() has the same function signature as fprintf() and newer
gcc warns when a non-constant string is fed as the format

Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.c

@@ -741,7 +741,7 @@ static void wt_shortstatus_other(int null_termination, struct string_list_item *
 		struct strbuf onebuf = STRBUF_INIT;
 		const char *one;
 		one = quote_path(it->string, -1, &onebuf, s->prefix);
-		color_fprintf(s->fp, color(WT_STATUS_UNTRACKED, s), sign);
+		color_fprintf(s->fp, color(WT_STATUS_UNTRACKED, s), "%s", sign);
 		printf(" %s\n", one);
 		strbuf_release(&onebuf);
 	}