* [ fix ] Fix returned status of the `system` function
* [ re #238 ] Fix program error condition of `git diff` call in `Golden`
According to documentation, not only negative exit code means error
To be able to eventually refactor/extend `system` function: to be
able to specify a directory, environment variables, specify arguments
as array etc. Ideally it should be something like Rust
[`std::process::Command`](https://doc.rust-lang.org/std/process/struct.Command.html).
