parse_pathspec: support stripping/checking submodule paths
PATHSPEC_SYMLINK_LEADING_PATH and _STRIP_SUBMODULE_SLASH_EXPENSIVE are
respectively the alternate implementation of
pathspec.c:die_if_path_beyond_symlink() and
pathspec.c:check_path_for_gitlink(). They are intended to replace
those functions when builtin/add.c and builtin/check-ignore.c are
converted to use parse_pathspec.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>