Re: fatal: ambiguous argument 'HEAD^': unknown revision or path not in the working tree.

Jeff King [off-list ref] writes:

The "simplest" way to do that is:

  rm .git/refs/heads/master

but that is not guaranteed to work in future versions of git, and I
wouldn't recommend it in general.

Yeah, "git checkout -b" has a stronger sibling "git checkout -B"
(i.e. I know that the named branch already exists, so this is not a
request to create a new one, but to repoint that existing one), but
there is no analogue for "git checkout --orphan".  That could be
worked around with something like this though:

    $ git checkout --orphan hu
    $ git branch -D master
    $ git checkout --orphan master

As I agree with your "amending is better", I do not think allowing
an easier access to such a feature is urgent or important, though.

Probably a better workflow is to create the commit state you _do_ want
(change files, "git add" them, etc), and then run "git commit --amend"
to replace the bad commit.