restore: disable overlay mode by default

restore: disable overlay mode by default

Overlay mode is considered confusing when the command is about
restoring files on worktree. Disable it by default. The user can still
turn it on, or use 'git checkout' which still has overlay mode on by
default.

While at it, make the check in checkout_branch() stricter. Neither
--overlay or --no-overlay should be accepted in branch switching mode.

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