Add struct pathspec

Add struct pathspec

The old pathspec structure remains as pathspec.raw[]. New things are
stored in pathspec.items[]. There's no guarantee that the pathspec
order in raw[] is exactly as in items[].

raw[] is external (source) data and is untouched by pathspec
manipulation functions. It eases migration from old const char ** to
this new struct.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>