BUG： Spare checkout would checkout all files.

[wethura]

Backgrounp: I try to clone the sub-dir in a git repo, and i found Chekout with SparseCheckoutDirectories fit me. But when i use it to achive my goal, i found it's useless. i found it work to checkout the specify dirs only when i clone with NoCheckout=false and checkout the same tag/branch like clone.

Reproduce:

code-01:

        fs := memfs.New()
	r, err := gitv5.Clone(memory.NewStorage(), fs, &gitv5.CloneOptions{
		URL: "{{repo url}}",
		ReferenceName:   plumbing.NewTagReferenceName(tag),
	})
	require.NoError(t, err)

	worktree, err := r.Worktree()
	require.NoError(t, err)

	err = worktree.Checkout(&gitv5.CheckoutOptions{
		SparseCheckoutDirectories: []string{"test"},
		Branch:                    plumbing.NewBranchReferenceName(branch),
	})

	require.NoError(t, err) // you can found all files are checkouted.

code-02:

        fs := memfs.New()
	r, err := gitv5.Clone(memory.NewStorage(), fs, &gitv5.CloneOptions{
		URL: "{{repo url}}",
		ReferenceName:   plumbing.NewTagReferenceName(tag),
                NoCheckout: true,
	})
	require.NoError(t, err)

	worktree, err := r.Worktree()
	require.NoError(t, err)

	err = worktree.Checkout(&gitv5.CheckoutOptions{
		SparseCheckoutDirectories: []string{"test"},
		Branch:                    plumbing.NewTagReferenceName(tag),
	})

	require.NoError(t, err) // you can found all files are checkouted.