Output of comparing two Japanese words is unreadable

## Summary

When I want to compare two Japanese words and there is diff, the returned value of `Diff` method is unreadable.

## Detail

Here is an example.

```golang
func Echo() string {
return "プライベート ブランド シャツ"
}
```

test code

```golang
func TestEcho(t *testing.T) {
expected := "プライベート ブランド ジャケット"
actual := Echo()
if diff := cmp.Diff(expected, actual); diff != "" {
t.Errorf("%s result mismatch (-want, +got):\n%s", t.Name(), diff)
}
}
```

`cmp.Diff` produces

```bash
--- FAIL: TestEcho (0.00s)
main_test.go:13: TestEcho result mismatch (-want, +got):
strings.Join({
"プライベート ブランド \xe3\x82",
- "\xb8ャケット",
+ "\xb7ャツ",
}, "")
FAIL
FAIL cmpbug 0.347s
FAIL
```

the above output is not readable. I expect the following output

```bash
❯ go test ./...
--- FAIL: TestEcho (0.00s)
main_test.go:13: TestEcho result mismatch (-want, +got):
  string(
"プライベート ブランド ",
-  "ジャケット",
+  "シャツ",
  )
FAIL
FAIL cmpbug 0.229s
FAIL
```

version info

- go: 1.19.2
- cmp: v0.5.9