rebase -r: do not (re-)generate root commits with `--root` *and* `--onto`

When rebasing a complete commit history onto a given commit, it is
pretty obvious that the root commits should be rebased on top of said
given commit.

To test this, let's kill two birds with one stone and add a test case to
t3427-rebase-subtree.sh that not only demonstrates that this works, but
also that `git rebase -r` works with merge strategies now.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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