GitHub: Stop treating Cargo.lock as a generated file.
GitHub: Stop treating Cargo.lock as a generated file.

We do want to inspect the changes to Cargo.lock, hiding the diff by default
would miss important details like
https://github.com/rust-lang/rust/pull/50629#discussion_r187556602

graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust