:param repo: the repo full name, ``{owner}/{project}``.
            e.g. ``buildbot/buildbot``

def _get_commit_msg(self, repo, sha):
        '''
        :param repo: the repo full name, ``{owner}/{project}``.
            e.g. ``buildbot/buildbot``
        '''
        headers = {
            'User-Agent': 'Buildbot'
        }
        if self._token:
            headers['Authorization'] = 'token ' + self._token

        url = '/repos/{}/commits/{}'.format(repo, sha)
        http = yield httpclientservice.HTTPClientService.getService(
            self.master, self.github_api_endpoint, headers=headers,
            debug=self.debug, verify=self.verify)
        res = yield http.get(url)
        data = yield res.json()
        msg = data.get('commit', {'message': 'No message field'})['message']
        return msg