pathspec: catch prepending :(prefix) on pathspec with short magic

:(prefix) is in the long form. Suppose people pass :!foo with '!' being the short form of magic 'bar', the code will happily turn it to :(prefix..)!foo, which makes '!' part of the path and no longer a magic. The correct form must be ':(prefix..,bar)foo', but as so far we haven't had any magic in short form yet (*), the code to convert from short form to long one will be inactive anyway. Let's postpone it until a real short form magic appears. (*) The short form magic '/' is a special case and won't be caught by this die(), which is correct. When '/' magic is detected, prefixlen is set back to 0 and the whole "if (prefixlen..)" block is skipped. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>