Working directory in inconsistent state after `./reinit`

In particular, .git contains the meta-data of the wrapper repository, but the rest of the directory contains the (patched) working tree of the wrapped repository.

It would be better if .git contained the meta-data of the wrapped repository too, so that I can look up the commit I end up being on in that repository.

See also: https://gitlab.dune-project.org/joe/pacxx-docker/merge_requests/53#note_48408