t7900-maintenance: test for magic markers

When we insert our "BEGIN" and "END" markers into the cron table, it's
so that a Git version from many years into the future would be able to
identify this region in the cron table. Let's add a test to make sure
that these markers don't ever change.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7900-maintenance.sh

@@ -446,6 +446,15 @@ test_expect_success 'start preserves existing schedule' '
 	grep "Important information!" cron.txt
 '
 
+test_expect_success 'magic markers are correct' '
+	grep "GIT MAINTENANCE SCHEDULE" cron.txt >actual &&
+	cat >expect <<-\EOF &&
+	# BEGIN GIT MAINTENANCE SCHEDULE
+	# END GIT MAINTENANCE SCHEDULE
+	EOF
+	test_cmp actual expect
+'
+
 test_expect_success 'stop preserves surrounding schedule' '
 	echo "Crucial information!" >>cron.txt &&
 	GIT_TEST_CRONTAB="test-tool crontab cron.txt" git maintenance stop &&