Hi Maxim,

hmm, git worktree can fail if the commit already is checked out somewhere (for
example if you invoke make update-guix-package twice in a row), or if the user
used git worktree on that repo for that commit for other purposes.  That would
mean that

  make update-guix-package

would fail in weird undocumented ways again.  Please please let's document
stuff at least.

Also, why not just fail when there's uncommitted stuff?

This patch looks like it goes to quite some length to enable you to build a
guix package of committed stuff only (which is NOT what your working copy is
actually like).  Is there a use case for that?  Sounds weird to me.

Even if there's a use case for that, please add a warning if there are
uncommitted changes that are now not included in the "guix" package.

Other than that, okay.

>#FIXME: This doesn't work (recursion?)
>./pre-inst-env guix build guix --with-git-url=guix=file://$PWD

Why doesn't it work?  That sounds like a big limitation--that basically means
you can't test with local-only commits, you'd always have to push.  We
should find out why this doesn't work and fix it.