p3400: replace calls to `git checkout -b' by `git checkout -B'

p3400: replace calls to `git checkout -b' by `git checkout -B'

p3400 makes a copy of the current repository to test git-rebase
performance, and creates new branches in the copy with `git checkout
-b'.  If the original repository has branches with the same name as the
script is trying to create, this operation will fail.

This replaces these calls by `git checkout -B' to force the creation and
update of these branches.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>