Decide the name of the default branch given the file and the configuration

        :param file: File with informations about it
        :return: Branch Name

def default_branch(self, file):
        """ Decide the name of the default branch given the file and the configuration

        :param file: File with informations about it
        :return: Branch Name
        """
        if isinstance(self.__default_branch__, str):
            return self.__default_branch__
        elif self.__default_branch__ == GithubProxy.DEFAULT_BRANCH.NO:
            return self.master_upstream
        else:
            return file.sha[:8]