t4150: am.messageid really adds the message id

Since a078f73 (git-am: add --message-id/--no-message-id, 2014-11-25),
the am.messageid setting determines whether the --message-id option is
set by default.

Add a test for this.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4150-am.sh

@@ -563,6 +563,18 @@ test_expect_success 'am --message-id really adds the message id' '
 	test_cmp expected actual
 '
 
+test_expect_success 'am.messageid really adds the message id' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout HEAD^ &&
+	test_config am.messageid true &&
+	git am patch1.eml &&
+	test_path_is_missing .git/rebase-apply &&
+	git cat-file commit HEAD | tail -n1 >actual &&
+	grep Message-Id patch1.eml >expected &&
+	test_cmp expected actual
+'
+
 test_expect_success 'am --message-id -s signs off after the message id' '
 	rm -fr .git/rebase-apply &&
 	git reset --hard &&