Declare both git-switch and git-restore experimental

Declare both git-switch and git-restore experimental

These two commands are basically redesigned git-checkout. We will not
have that many opportunities to redo (because we run out of verbs, and
that would also increase maintenance cost).

To play it safe, let's declare the two commands experimental in one or
two releases. If there is a serious flaw in the UI, we could still fix
it. If everything goes well and nobody complains loudly, we can remove
the experimental status by reverting this patch.

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