t3404: todo list with commented-out commands only aborts

If the todo list generated by `--make-script` is empty,
complete_action() writes a noop, but if it has only commented-out
commands, it will abort with the message "Nothing to do", and does not
launch the editor.  This adds a new test to ensure that
complete_action() behaves this way.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -75,6 +75,16 @@ test_expect_success 'rebase --keep-empty' '
 	test_line_count = 6 actual
 '
 
+cat > expect <<EOF
+Nothing to do
+EOF
+
+test_expect_success 'rebase -i with empty HEAD' '
+	set_fake_editor &&
+	test_must_fail env FAKE_LINES="1 exec_true" git rebase -i HEAD^ >actual 2>&1 &&
+	test_i18ncmp expect actual
+'
+
 test_expect_success 'rebase -i with the exec command' '
 	git checkout master &&
 	(