It would actually be a nice to build an optional warning feature into `git` itself. You could download some known common issues and the script would generate warnings if you checked in corresponding code. E.g.

    git add id_rsa
    
    WARNING: You may have just staged a private key.

or better

    echo '{"password": "mypassword"}' > config.json
    git add config.json

    WARNING: You may have just staged a password.