t4202 (log): Replace '<git-command> || :' with test_might_fail

Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4202-log.sh

@@ -393,7 +393,7 @@ test_expect_success 'log --graph with merge' '
 '
 
 test_expect_success 'log.decorate configuration' '
-	git config --unset-all log.decorate || :
+	test_might_fail git config --unset-all log.decorate &&
 
 	git log --oneline >expect.none &&
 	git log --oneline --decorate >expect.short &&