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.