'Compare two commits.
:param str base: (required), base for the comparison
:param str head: (required), compare this against base
:returns: :class:`Comparison <github3.repos.comparison.Comparison>` if
successful, else None'

def compare_commits(self, base, head):

    url = self._build_url(u'compare', ((base + u'...') + head), base_url=self._api)
    json = self._json(self._get(url), 200)
    return self._instance_or_null(Comparison, json)
