rebase-interactive.c: silence format-zero-length warnings

Fixes the following warnings:

rebase-interactive.c: In function ‘edit_todo_list’:
rebase-interactive.c:137:38: warning: zero-length gnu_printf format string [-Wformat-zero-length]
    write_file(rebase_path_dropped(), "");
rebase-interactive.c:144:37: warning: zero-length gnu_printf format string [-Wformat-zero-length]
   write_file(rebase_path_dropped(), "");

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

rebase-interactive.c

@@ -134,14 +134,14 @@ int edit_todo_list(struct repository *r, struct todo_list *todo_list,
 
 	if (incorrect) {
 		if (todo_list_check_against_backup(r, new_todo)) {
-			write_file(rebase_path_dropped(), "");
+			write_file(rebase_path_dropped(), "%s", "");
 			return -4;
 		}
 
 		if (incorrect > 0)
 			unlink(rebase_path_dropped());
 	} else if (todo_list_check(todo_list, new_todo)) {
-		write_file(rebase_path_dropped(), "");
+		write_file(rebase_path_dropped(), "%s", "");
 		return -4;
 	}