git-up: use `HEAD` instead of `head`.
git-up: use `HEAD` instead of `head`.

Using `head` results in the following error:

    % git rev-parse head
    head
    fatal: ambiguous argument 'head': unknown revision or path not in the
    working tree.
    Use '--' to separate paths from revisions

scorpiovn/dotfiles,scorpiovn/dotfiles,blueyed/dotfiles,scorpiovn/dotfiles,blueyed/dotfiles,scorpiovn/dotfiles,blueyed/dotfiles,blueyed/dotfiles,blueyed/dotfiles,scorpiovn/dotfiles