switch: reject "do nothing" case

switch: reject "do nothing" case

"git checkout" can be executed without any arguments. What it does is
not exactly great: it switches from HEAD to HEAD and shows worktree
modification as a side effect.

Make switch reject this case. Just use "git status" if you want
that side effect. For switch, you have to either

- really switch a branch
- (explicitly) detach from the current branch
- create a new branch

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