t1300-repo-config: make it resilient to being run via 'sh -x'

One way to diagnose broken regression tests is to run the test
script using 'sh -x t... -i -v' to find out which call actually
demonstrates the symptom.

Hence it is pretty counterproductive if the test script behaves
differently when being run via 'sh -x', in particular when using
test_cmp or test_i18ncmp on redirected stderr.  A more recent way
"sh tXXXX -i -v -x" has the same issue.

So let's use test_i18ngrep (as suggested by Jonathan Nieder) instead of
test_cmp/test_i18ncmp to verify that stderr looks as expected.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -699,17 +699,13 @@ test_expect_success 'invalid unit' '
 	echo 1auto >expect &&
 	git config aninvalid.unit >actual &&
 	test_cmp expect actual &&
-	cat >expect <<-\EOF &&
-	fatal: bad numeric config value '\''1auto'\'' for '\''aninvalid.unit'\'' in file .git/config: invalid unit
-	EOF
 	test_must_fail git config --int --get aninvalid.unit 2>actual &&
-	test_i18ncmp expect actual
+	test_i18ngrep "bad numeric config value .1auto. for .aninvalid.unit. in file .git/config: invalid unit" actual
 '
 
 test_expect_success 'invalid stdin config' '
-	echo "fatal: bad config line 1 in standard input " >expect &&
 	echo "[broken" | test_must_fail git config --list --file - >output 2>&1 &&
-	test_cmp expect output
+	test_i18ngrep "bad config line 1 in standard input" output
 '
 
 cat > expect << EOF