Improve searching for XXX in tidy script (#3303)

Few places where previous version of tidy script cannot find XXX:
* inside one-line comment preceding by a few spaces;
* inside multiline comments (now it finds it if multiline comment starts
on the same line with XXX).

Change occurences of XXX found by new tidy script.

src/test/bench/shootout-threadring.rs

@@ -15,7 +15,7 @@ use std::os;
 fn start(n_tasks: int, token: int) {
     let (tx, mut rx) = channel();
     tx.send(token);
-    //  XXX could not get this to work with a range closure
+    //  FIXME could not get this to work with a range closure
     let mut i = 2;
     while i <= n_tasks {
         let (tx, next_rx) = channel();