git checkout -b: allow switching out of an unborn branch

git checkout -b: allow switching out of an unborn branch

Running "git checkout -b another" immediately after "git init" when you do
not even have a commit on 'master' fails with:

    $ git checkout -b another
    fatal: You are on a branch yet to be born

This is unnecessary, if we redefine "git checkout -b $name" that does not
take any $start_point (which has to be a commit) as "I want to check out a
new branch $name from the state I am in".

Signed-off-by: Junio C Hamano <gitster@pobox.com>