This library just reads specified file content and returns it.
If you use Capistrano 3 to deploy app, The REVISION file is located to your application's root directory.
This means it might help you to see the current revision which project now refers to like this:
$retriever = new \Issei\ScmRevisionNumberRetriever('/path/to/your-project/REVISION');
var_dump($retriever->getCurrentRevisionNumber()); // content of "/path/to/your-project/REVISION"