t5520: use better test to check stderr output

Checking stderr output using test_i18ncmp may lead to test failure as
some shells write trace output to stderr when run under 'set -x'.

Use test_i18ngrep instead of test_i18ncmp.

Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -317,15 +317,13 @@ test_expect_success 'pull --rebase --no-autostash & rebase.autostash unset' '
 '
 
 test_expect_success 'pull --autostash (without --rebase) should error out' '
-	test_must_fail git pull --autostash . copy 2>actual &&
-	echo "fatal: --[no-]autostash option is only valid with --rebase." >expect &&
-	test_i18ncmp actual expect
+	test_must_fail git pull --autostash . copy 2>err &&
+	test_i18ngrep "only valid with --rebase" err
 '
 
 test_expect_success 'pull --no-autostash (without --rebase) should error out' '
-	test_must_fail git pull --no-autostash . copy 2>actual &&
-	echo "fatal: --[no-]autostash option is only valid with --rebase." >expect &&
-	test_i18ncmp actual expect
+	test_must_fail git pull --no-autostash . copy 2>err &&
+	test_i18ngrep "only valid with --rebase" err
 '
 
 test_expect_success 'pull.rebase' '