ngtip: rely on topicmap for 'ngtip' ngtip is an old function that predates the introduction of a more proper topic aware 'topicmap'. We replace the old code with a call to the new one. This change set function return type and have minor effect in a couple of place.

topic: list the number of 'behind' changeset when --verbose is used Displaying more information in the topic list is useful, we continue with the number of 'behind' changesets. This 'behind' count the number of new changesets on the default rebase destination. This will highlight topics that need rebasing.

destination: honor the sourceset argument of _destmerge Mercurial gained a "sourceset" argument to compute the merge destination of any changeset (the one in the 'sourceset'). This now works for the version in topic too. The support is quite basic (because we pick only one changeset from the sourceset) but that's a start.