println!() breaks incr. comp.

The following test, when added to the `incremental` test suite, fails:

```
// revisions:rpass1 rpass2

#![feature(rustc_attrs)]
#![rustc_partition_reused(module="println", cfg="rpass2")]

fn main() {
    println!("hello world");
}
```

Something causes the hash for `main` to be different in the two revisions.
