t7411: become resilient to GETTEXT_POISON

The concerned test greps the error message in git_parse_source() which
contains "bad config line %d in submodule-blob %s".

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7411-submodule-config.sh

@@ -89,7 +89,7 @@ test_expect_success 'error message contains blob reference' '
 			HEAD b \
 			HEAD submodule \
 				2>actual_err &&
-		grep "submodule-blob $sha1:.gitmodules" actual_err >/dev/null
+		test_i18ngrep "submodule-blob $sha1:.gitmodules" actual_err >/dev/null
 	)
 '