File written to cfg_attr path is not formatted

[rchaser53]

related: #3933, #3956

The file written to cfg_attr path is not formatted. It seems to be degraded since a178776.
(It works until 61c1601)

[rchaser53]

below is an example.

// lib.rs
#[cfg_attr(windows, path = "imp-a.rs")]
#[cfg_attr(not(windows), path = "imp-b.rs")]
mod imp;
pub use imp::A;

// imp-a.rs
    pub struct A;  // This is not formatted

// imp-b.rs
    pub struct B;  // This is also not formatted

[rchaser53]

I forgot to add a config file to ignore files except for lib.rs in the last PR. (like this)
So every file is the target for rustfmt. This is the one reason why the test failed to find this degrading.

But there is another problem if I add the config file. The result is the same if I add the config file.
When there are a config file and logic works correctly, the test passes. It's ok.
When there are a config file and logic doesn't work, the test formats and compares lib.rs only. It passes.
When there is not a config file, every file is formatted. It passes.
All pattern is a success every time. I think need to add another way to catch the degrade.

[rchaser53]

It's not degrading. The default behavior is changed, so it needs to add the option -r for recursive.
I didn't add the option. Sorry.