bookmarks: resolve divergent bookmarks when moving active bookmark forward This patch resolves divergent bookmarks between the current active bookmark MARK and the new destination. This situation can arise when pulling new changesets, abandoning your current changesets actively bookmarked with MARK via strip, and then doing a bare update. The non-divergent but active bookmark MARK is then moved to a common ancestor of the new changesets and the abandoned changesets. Test coverage is added.