t3404: directly test the behavior of interest

t3404.3 is a simple test added by commit d078c39106 ("t3404: todo list
with commented-out commands only aborts", 2018-08-10) which was designed
to test a todo list that only contained commented-out commands.  There
were two problems with this test: (1) its title did not reflect the
purpose of the test, and (2) it tested the desired behavior through a
side-effect of other functionality instead of directly testing the
desired behavior discussed in the commit message.

Modify the test to directly test the desired behavior and update the
test title.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -72,15 +72,16 @@ test_expect_success 'rebase --keep-empty' '
 	test_line_count = 6 actual
 '
 
-test_expect_success 'rebase -i with empty HEAD' '
+test_expect_success 'rebase -i with empty todo list' '
 	cat >expect <<-\EOF &&
 	error: nothing to do
 	EOF
 	(
 		set_fake_editor &&
-		test_must_fail env FAKE_LINES="1 exec_true" \
-			git rebase -i HEAD^ >actual 2>&1
+		test_must_fail env FAKE_LINES="#" \
+			git rebase -i HEAD^ >output 2>&1
 	) &&
+	tail -n 1 output >actual &&  # Ignore output about changing todo list
 	test_i18ncmp expect actual
 '