Add tests for rebase -i --root without --onto

Add tests for rebase -i --root without --onto

Test for likely breakages in t3404, including successful reordering of
non-conflicting changes with a new root, correct preservation of commit
message and author in a root commit when it is squashed with the
sentinel, and presence of the sentinel following a conflicting
cherry-pick of a new root.

Remove test_must_fail for git rebase --root without --onto from t3412 as
this case will now be successfully handled by an implicit git rebase -i.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>