D6168: readline: provide styled prompt to readline (issue6070)

[Yuya Nishihara]

>   Oh, right... I saw that and that was part of why I didn't mail earlier :D  I think this is just an "expected" difference in behavior,

No. It provides metadata to command-server clients (e.g. GUI/editor frontend.)
Basically we'll need:

```
if usereadline:
    rawinput(labeled_prompt)
else:
    write_prompt_as_before()
    fin.readline()
```

In chg session, stdio is directly accessible from the server process. No
command-server protocol is used after the connection is established.