Make chdir to parent directory obvious

This notation resolves two issues with single step:

1.  It makes obvious that we go to upper directory.
    dirname(__DIR__) looks confusing after a quick reading.
    It made me think that it is some kind of protection
    to surely get directory path on every OS.

2.  Resolves symlinks.
    realpath() makes a syscall to get a physical path.
    It can help to avoid errors in some rare modules which doesn't
    work correctly with symlinks.
    I can't recall exactly but there were such cases in my practice
    when application couldn't find the directory it operates on
    because it was symlinked.