running dune in a git repository without any commit fails with a git error

[hannesm]

Expected Behavior

successful build of a binary

Actual Behavior

# dune build no.exe
         git (internal) (exit 128)
/usr/local/bin/git rev-parse HEAD > /tmp/dune1dde43.output
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Reproduction

# mkdir /tmp/foo
# cd /tmp/foo
# touch no.ml
# echo "(executable (name no))" > dune
# dune build no.exe
--> success (as expected)
# dune clean
# git init
# dune build no.exe
         git (internal) (exit 128)
/usr/local/bin/git rev-parse HEAD > /tmp/dune1dde43.output
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
--> here I expected it to "just work"

Specifications

• Version of dune (output of dune --version):
  2.7.1
• Version of ocaml (output of ocamlc --version)
• Operating system (distribution and version):

[rgrinberg]

#4441 fixes this.