obsolete: add a divergent method on context The same we have `unstable` and `bumped`. Convenient method to access troubles information in general may land later. This get actual use and testing in the next changesets.

obsolete: add revset and test for divergent changesets This changesets add a new `divergent()` revset similar to `unstable()` and `bumped()` one. Introducting this revset allows actuall test of the divergent detection.

obsolete: detect divergent changesets Divergent changeset are final successors (non obsolete) of a changeset who compete with another set of final successors for this same changeset. For example if you have two obsolescence markers A -> B and A -> C, B and C are both "divergent" because they compete to be the one true successors of A. Public revision can't be divergent. This function is used and tested in the next changeset.