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.

topic: list the number of changesets when --verbose is used Displaying more information in the topic list is useful, we start with the most obvious: number of non-obsolete changeset in the topic.

topic: add color for the active topic We reuse the same color that the one used for the active bookmark.

topic: add formatter support Using formatter to output the list of topic will add support for color and richer templater. We add some test for output control. We introduce a dedicated test to help with coming more complexe output.

topic: extract the code listing all topics The code will become more complexe with formatter support and additional data in the verbose output. We extract the code in its own function first.