[PATCH 0 of 4] Fix and generate revsets help dynamically

[Patrick Mezard]

By generating revsets predicates helps dynamically we are able to simplify
the way extensions register them while integrating their help seamlessly in the
existing topic.

The only thing I suspect might break here is the revset rewriter function
registration in help command. I don't know how tortoisehg or similar
applications extract and display help topics right now. It might be useful to
move this registration in a dedicated loaddoc() function somewhere in
cmdutil.py for instance.

Also, we might want to change the extensions revset help again to specify they
come from extensions.