This is necessary to ensure Cargo knows when to rebuild. Normally
.gitignore would be enough to exclude these from Cargo's freshness
calculation, but https://github.com/rust-lang/cargo/issues/1729 prevents
this currently. This is the new, correct way to do these thigns, just
like the style crate does.
Source-Repo: https://github.com/servo/servo
Source-Revision: ff59e1b8de67f1988522d6b6e6c4adda957fad5b
ed7664058c