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

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>