Show a failure of rebase -p if the merge had a conflict

Show a failure of rebase -p if the merge had a conflict

This extends t3409-rebase-preserve-merges by a case where the merge that
is rebased has a conflict. Therefore, the rebase stops and expects that
the user resolves the conflict. However, currently rebase --continue
fails because .git/rebase-merge/author-script is missing.

The test script had allocated two identical clones, but only one of them
(clone2) was used. Now we use both as indicated in the comment. Also,
two instances of && was missing in the setup part.

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