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.

---
Noticed while I was working on replacing 'git merge' by 'git cherry-pick' in the rebase -p code path.

 t/t3411-rebase-preserve-around-merges.sh | 1 +
 1 file changed, 1 insertion(+)

b/t/t3411-rebase-preserve-around-merges.sh
index ace8e54..dc81bf2 100755

@@ -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 &&

1.7.10.2.529.g0c18cfd

Makes sense to me. Thanks, Johannes.

- Stephen