create non-writable-file precommit file

[Uzlopak]

transplantation from observing/pre-commit#161

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[jsumners]

I agree that this looks like a simple fix. But we really need to implement a robust integration test suite before we try things like this.