pager: move most help to a new help topic and deprecate extension

dispatch: rearrange 'unknown command' code to better employ pager dispatch calls help like a ninja if you give it a truly unknown command, and that might want to be paged. If it gets paged, then the 'hg: unknown command' text gets eaten by a grue, unless we call the pager first. This change rearranges the codepaths so we can safely only invoke the pager in the case where we'll have long output from the help command code, rather than just a short message like "did you mean stat instead of start" or "fetch is provided by the fetch extension".

help: move rst formatting of help documents into help.py This slight refactor will help me improve the 'unknown command' experience in our new glorious pager future.

outgoing: avoid running pager until we're actually showing changes Consistent with the new behavior of incoming in the previous patch.