--allow-empty is no longer completable because it's a hidden option
since 4741edd549 (Remove deprecated OPTION_BOOLEAN for parsing arguments
- 2013-08-03)
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>