D5800: config: introduce a new value for ui.relative-paths getting old behavior

[Yuya Nishihara]

>   > What I thought confusing is `scmutil.getuipathfn(ctx.repo(), legacyvalue=True)`
>   >  in https://phab.mercurial-scm.org/D5801. "What does the `True` mean? relative, absolute, or a complete
>   >  different stuff?"
>   
>   Same reason it's confusing, I believe: it's unclear if "legacyvalue=True" means "use the legacy value" (incorrect) or "for the legacy value, use the value True" (correct). I was hoping the "value" part would clarify that, but I agree that it's still not clear.  I think you're also saying that the fact that the function deals with producing a cwd-relative or absolute (well, repo-relative) is also not clear and I agree with that too.

Yes. `force/legacyvalue` doesn't provide what the `value` means, which is my
point. I also get your point. Naming is hard.

> Maybe "usedtoberelative=True"?

As a non-native speaker, a single word "legacy" is easier to parse than a
phrase "used to be".