Generate a text report for a diff.

def html(self, diff, f):
        """
        Generate a text report for a diff.
        """
        env = Environment(loader=PackageLoader('clan', 'templates'))

        template = env.get_template('diff.html')

        def number_class(v):
            if v is None:
                return ''

            if v > 0:
                return 'positive'
            elif v < 0:
                return 'negative'

            return ''

        context = {
            'diff': diff,
            'GLOBAL_ARGUMENTS': GLOBAL_ARGUMENTS,
            'format_comma': format_comma,
            'format_duration': format_duration,
            'number_class': number_class

        }

        f.write(template.render(**context).encode('utf-8'))