checkout: learn to respect checkout.guess

checkout: learn to respect checkout.guess

The current behavior of git checkout/switch is that --guess is currently
enabled by default. However, some users may not wish for this to happen
automatically. Instead of forcing users to specify --no-guess manually
each time, teach these commands the checkout.guess configuration
variable that gives users the option to set a default behavior.

Teach the completion script to recognize the new config variable and
disable DWIM logic if it is set to false.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>