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".