tests: have a more elaborated test for _destrebase The previous repo had a single lineage and a single head, pretty boring for rebase related testing.

histedit: break _histedit function into smaller pieces We add _getgoal, _validateargs. This is a part of bigger effort to refactor histedit. Initial steps are to break _histedit function into smaller pieces which will supposedly be more understandable. After this is done, I will have a better understanding of how histedit works and apply that to fix issue4800.