Add a reproduction case for issue #4429 (#4430)

Signed-off-by: Craig Ferguson <me@craigfe.io>
Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

Author: craigfe

test/blackbox-tests/test-cases/github4429.t/run.t

@@ -0,0 +1,24 @@
+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
+
+At the moment Dune fails, which is bad:
+
+  $ dune exec ./main.exe 2>&1 | sed 's/.*\/git/{{ git }}/; s/> .*.output/> {{ output_file }}/g' 
+           git (internal) (exit 128)
+  {{ git }} describe --always --dirty > {{ output_file }}
+  fatal: bad revision 'HEAD'
+