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

[Yuya Nishihara]

>   I was also thinking we could do that, but it feels like a workaround to me. I think of bookmarks and tags as pointing to specific commits. I think of branches as being many commits. `hg help revisions` says this:
>   
>     Any other string is treated as a bookmark, tag, or branch name. A bookmark
>     is a movable pointer to a revision. A tag is a permanent name associated
>     with a revision. A branch name denotes the tipmost open branch head of
>     that branch - or if they are all closed, the tipmost closed head of the
>     branch. Bookmark, tag, and branch names must not contain the ":"
>     character.
>   
>   It specifically mentions bookmarks, tags, and branches, and it describes how each name resolves to a commit. To me, that makes it sound like some symbols can resolve to multiple commits. And indeed, revset aliases can already resolve to multiple commits.

I read it implying that any sort of symbol resolves to a single revision since
all three core "names" work in such way. I'm not against adding multi-node
symbol resolution because it seems useful, but I agree with Sean that it would
introduce inconsistency.

I have no idea about the `namemap()` API.