switch: implicit dwim, use --no-guess to disable it

switch: implicit dwim, use --no-guess to disable it

This is already the default in git-checkout. The real change in here is
just minor cleanup. The main excuse is to explain why dwim is kept default.

Contrary to detach mode that is easy to get into and confusing to get
back out. Automatically creating a tracking branch often does not kick
in as often (you would need a branch of the same name on a remote). And
since the branch creation is reported clearly, the user should be able
to undo/delete it if it's unwanted.

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