help: merge section about uisetup() and extsetup() They are technically the same callback, called only once per process. The section name "ui setup" is confusing, so shouldn't be used.

help: add internals.extensions topic I think it's better to include the API overview in core as we now have the internals section in our help system. Retrieved from the wiki, and formatted as reST. Several wiki links are removed since they are invalid in the help. The sections about example extension are removed at all as they seemed too verbose. https://www.mercurial-scm.org/wiki/WritingExtensions

commandserver: send raw progress information to message channel This is pretty basic implementation to support GUI progress bar.