Return the relative path of setup.py to the git repo root.

def _get_subdirectory(cls, location):
        """Return the relative path of setup.py to the git repo root."""
        # find the repo root
        git_dir = cls.run_command(['rev-parse', '--git-dir'],
                                  show_stdout=False, cwd=location).strip()
        if not os.path.isabs(git_dir):
            git_dir = os.path.join(location, git_dir)
        root_dir = os.path.join(git_dir, '..')
        # find setup.py
        orig_location = location
        while not os.path.exists(os.path.join(location, 'setup.py')):
            last_location = location
            location = os.path.dirname(location)
            if location == last_location:
                # We've traversed up to the root of the filesystem without
                # finding setup.py
                logger.warning(
                    "Could not find setup.py for directory %s (tried all "
                    "parent directories)",
                    orig_location,
                )
                return None
        # relative path of setup.py to repo root
        if samefile(root_dir, location):
            return None
        return os.path.relpath(location, root_dir)