'special' is too generic and is used for two different purposes.
Replace it with 'match_slash' to indicate "**" pattern and 'negated'
for "[!...]" and "[^...]".
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>