t3428: restore coverage for "apply" backend

This test file assumes the "apply" backend is the default which is not
the case since 2ac0d6273f (rebase: change the default backend from "am"
to "merge", 2020-02-15). Make sure the "apply" backend is tested by
specifying it explicitly.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3428-rebase-signoff.sh

diff --git a/t/t3428-rebase-signoff.sh b/t/t3428-rebase-signoff.sh
index 133e541..1bebd1c 100755
--- a/t/t3428-rebase-signoff.sh
+++ b/t/t3428-rebase-signoff.sh
@@ -38,8 +38,8 @@
 
 # We configure an alias to do the rebase --signoff so that
 # on the next subtest we can show that --no-signoff overrides the alias
-test_expect_success 'rebase --signoff adds a sign-off line' '
-	git rbs HEAD^ &&
+test_expect_success 'rebase --apply --signoff adds a sign-off line' '
+	git rbs --apply HEAD^ &&
 	test_commit_message HEAD expected-signed
 '