t3427: accommodate for the `rebase --merge` backend having been replaced
Since
68aa495b590 (rebase: implement --merge via the interactive
machinery, 2018-12-11), the job of the old `--merge` backend is now
performed by the `--interactive` backend, too.
One consequence is that empty commits are no longer rebased by default.
Meaning that the test case that calls `git rebase -Xsubtree` (which used
to be handled by the `--merge` backend) now needs to ask explicitly for
the empty commit to be rebased.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>