Determines the revision against which the staged data ought to be checked.

    :returns: The revision.
    :rtype: :class:`str`

def get_against():
    """
    Determines the revision against which the staged data ought to be checked.

    :returns: The revision.
    :rtype: :class:`str`
    """
    global __cached_against
    if __cached_against is not None:
        return __cached_against

    status = subprocess.call(["git", "rev-parse", "--verify", "HEAD"],
                             stdout=open(os.devnull, 'w'),
                             stderr=subprocess.STDOUT)
    if not status:
        against = 'HEAD'
    else:
	# Initial commit: diff against an empty tree object
        against = '4b825dc642cb6eb9a060e54bf8d69288fbee4904'

    __cached_against = against
    return against