Someone on discord reported issues with UI tests.
This should make sure that git never automatically converts lineendings
for text files to `CRLF`. They should always be `LF` now.
Probably this means that we can stop using dos2unix for #3306, too.
Taken from [Rust's .gitattributes file](https://github.com/rust-lang/rust/blob/master/.gitattributes).
