`cleanGit` doesn't detect git worktrees correctly

[typetetris]

Take any repository using haskell.nix and cleanGit and add a new worktree like

git worktree add ../new-worktree <commitish>
cd ../new-worktree
nix-shell # or whatever you do to trigger `shellFor` or so

and you should see

trace: gitSource.nix: /<path-to/new-worktree does not seem to be a git repository

but git ls-files and the like would work.

[nomeata]

The latest version of gitSource.nix that I am using here internally has this logic to handle worktrees:

    git_dir =
      if builtins.pathExists ../.git/index
      then ../.git
      else # likely a git worktree, so follow the indirection
        let
          git_content = lines (readFile ./../.git);
          first_line = head git_content;
          prefix = "gitdir: ";
          ok = length git_content == 1 && has_prefix prefix first_line;
        in
          if ok
          then /. + remove_prefix prefix first_line
          else abort "gitSource.nix: Cannot parse ${toString ./../.git}";

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.