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>

t/t3411-rebase-preserve-around-merges.sh

@@ -56,6 +56,7 @@ test_expect_success 'squash F1 into D1' '
 # And rebase G1..M1 onto E2
 
 test_expect_success 'rebase two levels of merge' '
+	git checkout A1 &&
 	test_commit G1 &&
 	test_commit H1 &&
 	test_commit I1 &&