For the record, commit cb41c15827a2e910aa56fb5d1917ba8a085c95c7 by Kyle
(Cc’d) gives the ability to use the remote HEAD by default, which is
exactly what we need here.

Unfortunately, for the Guix repo at Savannah, I get:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix time-machine -- describe
guix time-machine: error: Git error: reference 'refs/remotes/origin/HEAD' not found
--8<---------------cut here---------------end--------------->8---

… but it works for <https://github.com/guix-mirror/guix>.

Presumably we have a server-side setup issue at Savannah?

Are there downsides, or cases where it might pick the wrong branch?

Thoughts?

Ludo’.