checkout: fix regression in checkout -b on intitial checkout

checkout: fix regression in checkout -b on intitial checkout

When doing a 'checkout -b' do a full checkout including updating the working
tree when doing the initial checkout. As the new test involves an filesystem
access, do it later in the sequence to give chance to other cheaper tests to
leave early. This fixes the regression in behavior caused by fa655d8411
(checkout: optimize "git checkout -b <new_branch>", 2018-08-16).

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>