D6183: copies: add config option for writing copy metadata to file and/or changset

[Yuya Nishihara]

>   > It might be nitpicky, but I think it's better to not embed `\0` into the
>   >  extras field. Almost all extras data are texts, and IIRC we regret that
>   >  transplant sources are stored in binary form.
>   
>   Why not? I picked \0 and \n because they won't appear in filenames, so it's convenient in that way.

I don't remember, but we do store even boolean value as text, not in binary
`\0`/`\1` form. `transplant_source` is the solo exception.

https://www.mercurial-scm.org/wiki/ChangesetExtra

And if we pick \0/\n separators, _string_escape() wouldn't be needed
at the encodecopies() layer.