evolve: test that target is not orig in _solveunstable() (issue6097) `newer` is the result of obsutil.successorssets() and can be [[orig.node()]], in which case later on in this function evolve will try to rebase orig onto orig, which is not correct. So let's just check this particular case. This fix doesn't cover cases when successorssets() result contains orig.node() not at [0][0]. Such cases need tests.