Fix tidy on Windows

[petrochenkov]

This is exactly the same issue as rust-lang/book#549, x.py test src/tools/tidy is failing again.
r? @steveklabnik@carols10cents

[carols10cents]

I don't have r here, but lgtm!

[steveklabnik]

Thanks!

[steveklabnik]

x.py test src/tools/tidy is failing again.

incidentally, how did you notice this? Did you try to update the submodule yourself?

[petrochenkov]

@steveklabnik
Default git settings on Windows is "checkout as \r\n, commit as \n", so this issue reproduced automatically when I updated the Rust repo and run tidy.
This can be solved by adding .gitattributes to the repo once, or changing settings to "checkout as \n" by all people testing Rust on Windows. Adding .gitattributes seems nicer and the main repo did this, but every time someone creates a new submodule and forgets to add .gitattributes the problem repeats.