Test the behaviour of Dune querying git when the repo has no commit.

Create a repository with no HEAD commit:

  $ git init --quiet

... and an executable that links `dune-build-info':

  $ echo "(lang dune 2.0)" > dune-project
  $ touch main.ml
  $ cat >dune <<EOF
  > (executable
  >  (name main)
  >  (libraries dune-build-info)
  >  (promote (until-clean)))
  > EOF

Dune handles this gracefully since #4441

  $ dune exec ./main.exe 2>&1