auto merge of #5579 : dbaupp/rust/rustc-typo-limit, r=catamorphism

Impose a limit so that the typo suggester only shows reasonable
suggestions (i.e. don't suggest `args` when the error is `foobar`).

A tiny bit of progress on #2281.

src/test/compile-fail/issue-2281-part1.rs

@@ -0,0 +1,13 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern: unresolved name: `foobar`.
+
+fn main(args: ~[str]) { debug!(foobar); }