git checkout: don't warn about unborn branch if -f is already passed

git checkout: don't warn about unborn branch if -f is already passed

I think it's unnecessary to warn that the checkout has been forced due to an
unborn current branch if -f has been explicitly passed.  For one project, I am
using git-new-workdir to create workdirs from a bare repository whose HEAD is
set to an unborn branch, and this warning started to irritate me.

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>