rebase -i -p: Fix --continue after a merge could not be redone

rebase -i -p: Fix --continue after a merge could not be redone

When a merge that has a conflict was rebased, then rebase stopped to let
the user resolve the conflicts. However, thereafter --continue failed
because the author-script was not saved. (This is rebase -i's way to
preserve a commit's authorship.) This fixes it by doing taking the same
failure route after a merge that is also taken after a normal cherry-pick.

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