Fix t3411.3 to actually rebase something

Fix t3411.3 to actually rebase something

The test intends to rebase a branchy history onto a later commit, but it
forgot to reset HEAD back to an earlier commit before it set up the side
branches. In the end, every "rebased" commit was only a fast-forward and
the 'rebase -p' did not change the commit graph at all. Insert the missing
checkout that moves to an earlier commit.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>