restore: make pathspec mandatory

restore: make pathspec mandatory

"git restore" without arguments does not make much sense when
it's about restoring files (what files now?). We could default to
either

    git restore .

or

    git restore :/

Neither is intuitive. Make the user always give pathspec, force the
user to think the scope of restore they want because this is a
destructive operation.

"git restore -p" without pathspec is an exception to this
because it really is a separate mode. It will be treated as running
patch mode on the whole worktree.

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