t1300: test "git config --get-all" more thoroughly

We check that we can "--get-all" a multi-valued variable,
but we do not actually confirm that the output is sensible.
Doing so reveals that it works fine, but this will help us
ensure we do not have regressions in the next few patches,
which will touch this area.

Signed-off-by: Jeff King <peff@peff.net>

t/t1300-repo-config.sh

@@ -258,8 +258,13 @@ test_expect_success 'ambiguous get' '
 	test_must_fail git config --get nextsection.nonewline
 '
 
-test_expect_success 'get multivar' '
-	git config --get-all nextsection.nonewline
+test_expect_success 'multi-valued get-all returns all' '
+	cat >expect <<-\EOF &&
+	wow
+	wow2 for me
+	EOF
+	git config --get-all nextsection.nonewline >actual &&
+	test_cmp expect actual
 '
 
 cat > expect << EOF