t3427: accommodate for the rebase --merge backend having been replaced

Since 68aa495 (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>

t/t3427-rebase-subtree.sh

@@ -83,10 +83,10 @@ test_expect_failure REBASE_P 'Rebase -Xsubtree --keep-empty --preserve-merges --
 '
 
 # FAILURE: fatal: Could not parse object
-test_expect_failure 'Rebase -Xsubtree --onto commit' '
+test_expect_failure 'Rebase -Xsubtree --keep-empty --onto commit' '
 	reset_rebase &&
 	git checkout -b rebase-onto to-rebase &&
-	git rebase -Xsubtree=files_subtree --onto files-master master &&
+	git rebase -Xsubtree=files_subtree --keep-empty --onto files-master master &&
 	verbose test "$(commit_message HEAD~2)" = "master4" &&
 	verbose test "$(commit_message HEAD~)" = "files_subtree/master5" &&
 	verbose test "$(commit_message HEAD)" = "Empty commit"