completion: improve sparse-checkout cone mode directory completion

completion: improve sparse-checkout cone mode directory completion

Use new __gitcomp_directories method to complete directory names in cone
mode sparse-checkouts. This method addresses the caveat of poor
performance in monorepos from the previous commit (by completing only one
level of directories).

The unusual character caveat from the previous commit will be fixed by the
final commit in this series.

Co-authored-by: Elijah Newren <newren@gmail.com>
Co-authored-by: Lessley Dennington <lessleydennington@gmail.com>
Signed-off-by: Lessley Dennington <lessleydennington@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>