pathspec: remove the deprecated get_pathspec function

pathspec: remove the deprecated get_pathspec function

Now that all callers of the old 'get_pathspec' interface have been
migrated to use the new pathspec struct interface it can be removed
from the codebase.

Since there are no more users of the '_raw' field in the pathspec struct
it can also be removed.  This patch also removes the old functionality
of modifying the const char **argv array that was passed into
parse_pathspec.  Instead the constructed 'match' string (which is a
pathspec element with the prefix prepended) is only stored in its
corresponding pathspec_item entry.

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