rebase --keep-empty -i: add test

There's some special code in rebase -i to deal
with --keep-empty.
Add test for this combination.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -67,6 +67,14 @@ test_expect_success 'setup' '
 SHELL=
 export SHELL
 
+test_expect_success 'rebase --keep-empty' '
+	git checkout -b emptybranch master &&
+	git commit --allow-empty -m "empty" &&
+	git rebase --keep-empty -i HEAD~2 &&
+	git log --oneline >actual &&
+	test_line_count = 6 actual
+'
+
 test_expect_success 'rebase -i with the exec command' '
 	git checkout master &&
 	(