destmerge: improve topic head computation and handling This rework get use two benefits: - heads are now computed on a "stabilized" state, - heads above the 'source' are now properly cleaned up,

evolve: extract the code copied from evolve in a submodule This will allow us to use that code in other module.

destmerge: honor the 'onheadcheck' argument This argument was introduced in Mercurial some time ago but the topic extension was never updated to take it in account. This is now fixed.

destination: use the correct abort class for error These errors have been in Mercurial long enough. Having them will make the coming changesets simpler.

topic: list the number of head when --verbose is used Displaying more information in the topic list is useful, we continue with the number of heads on the topics, this will help highlight the problem to users. We only display head if there is more than one.

topic: list the number of troubled changesets when --verbose is used Displaying more information in the topic list is useful, we continue with the number of troubled changesets. This will help people to spot topics that needs evolution, for example after a pull.