D3715: namespaces: allow namespaces whose symbols resolve to many nodes (API)

[Yuya Nishihara]

>   > This patch strikes me as a Seems-Like-A-Good-Idea-But-Could-Blowup type
>   >  of thing. So, what this patch does is conditionally change the behavior
>   >  of 'log -r' based on the type of object passed in.
>   
>   
>   No, it just allows namespaces to do that. As I said (or tried to say) in the commit message, `hg log -r stable` is protected by BC, so we can't change it. There should be no functional change from this patch.

How about adding syntax to resolve multinode namespace symbols?
`stable~`, for example, will be resolved to all revisions in the stable
branch.

Implementation wise, we can get rid of the `revsymbol()` hack, and extra
`repo[n]` lookup won't be needed. A possible drawback (other than the '~'
suffix itself) is we'll soon run out of shell-safe symbols.