return the string output of git desribe

def call_git_branch():
    """return the string output of git desribe"""
    try:
        with open(devnull, "w") as fnull:
            arguments = [GIT_COMMAND, 'rev-parse', '--abbrev-ref', 'HEAD']
            return check_output(arguments, cwd=CURRENT_DIRECTORY,
                                stderr=fnull).decode("ascii").strip()
    except (OSError, CalledProcessError):
        return None