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

Thomas Glanzmann [off-list ref] writes:

I see it. Last line. I tried to clone from a repository which itself had a
detached HEAD. Could that be the problem?

Ah, our mails crossed.

There is no reason to forbid cloning of a repository whose HEAD
is detached, but detached HEAD is a relatively recent invention,
and I would not be surprised if that was indeed the breakage,
nor nobody has hit this so far.

It's sufficiently late here so I do not think I would be able to
take a look at it tonight, though.  Sorry.

Just in case you need unblocking (although I think you know how
already), you should be able to continue with:

	$ git rev-parse v2.6.20^0 >.git/HEAD
        $ git read-tree HEAD
        $ git checkout-index -f -q -u -a

That would give you a normal detached state at v2.6.20; you
should be able to come back to your branches with usual "git
checkout".