hidden: extract the code generating "filtered rev" error for wrapping The goal is to help experimentation in extensions (ie: evolve) around more advance messages.

serve: add support for Mercurial subrepositories I've been using `hg serve --web-conf ...` with a simple '/=projects/**' [paths] configuration for awhile without issue. Let's ditch the need for the manual configuration in this case, and limit the repos served to the actual subrepos. This doesn't attempt to handle the case where a new subrepo appears while the server is running. That could probably be handled with a hook if somebody wants it. But it's such a rare case, it probably doesn't matter for the temporary serves. The main repo is served at '/', just like a repository without subrepos. I'm not sure why the duplicate 'adding ...' lines appear on Linux. They don't appear on Windows (see 594dd384803c), so they are optional. Subrepositories that are configured with '../path' or absolute paths are not cloneable from the server. (They aren't cloneable locally either, unless they also exist at their configured source, perhaps via the share extension.) They are still served, so that they can be browsed, or cloned individually. If we care about that cloning someday, we can probably just add the extra entries to the webconf dictionary. Even if the entries use '../' to escape the root, only the related subrepositories would end up in the dictionary.