Function set_submodule_repo_worktree 

pub fn set_submodule_repo_worktree(
    grit_bin: &Path,
    modules_dir: &Path,
    sub_worktree: &Path,
)

Sets core.worktree in the submodule repo at modules_dir via grit --git-dir.

Stores a path relative to modules_dir (e.g. ../../../sub1), matching C Git and test_git_directory_exists in the ported submodule tests.