If "include" option exists in "default.cfg",
        read the file(glob-match) in the directory.

def _merge_includes(self):
        """
        If "include" option exists in "default.cfg",
        read the file(glob-match) in the directory.
        """
        raw_include_path = self.get_global_include()
        if raw_include_path:
            abs_include_path = self._get_global_include_abs_path(
                raw_include_path
            )
            self._validate_global_include(abs_include_path)
            self.set_global_include(abs_include_path)

            for infile in glob.glob(abs_include_path):
                self.config.merge(
                    self._configobj_factory(infile=infile)
                )