pathspec: add copy_pathspec

pathspec: add copy_pathspec

Because free_pathspec wants to free "items" pointer in the pathspec
structure, a simple structure assignment is not enough if you want to
copy an existing pathspec into another.  Freeing the original will
damage the copy unless a deep copy is made.

Note that the strings in pathspec->items->match and the array
pathspec->raw[] are still shared between the original and the copy.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>