test: use test_i18ncmp for "Patch format detection failed" message

v1.7.8.5~2 (am: don't infloop for an empty input file, 2012-02-25)
added a check for the human-readable message "Patch format detection
failed." but we forgot to suppress that check when running tests with
git configured to write output in another language.

Noticed by running tests with GETTEXT_POISON=YesPlease.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4150-am.sh

@@ -527,7 +527,7 @@ test_expect_success 'am empty-file does not infloop' '
 	test_tick &&
 	{ git am empty-file > actual 2>&1 && false || :; } &&
 	echo Patch format detection failed. >expected &&
-	test_cmp expected actual
+	test_i18ncmp expected actual
 '
 
 test_done