dispatch: provide help for disabled extensions and commands Before a command is declared unknown, each extension in hgext is searched, starting with hgext.<cmdname>. If there's a matching command, a help message suggests the appropriate extension and how to enable it. Every extension could potentially be imported, but for cases like rebase, relink, etc. only one extension is imported. For the case of "hg help disabledext", if the extension is in hgext, the extension description is read and a similar help suggestion is printed. No extension import occurs.