histedit: add an experimental base action
This is a first (very simple) version of the histedit base action. It works well in common usecases like rebasing the whole stack and spliting the stack. I don't see any obvious edge cases - but probably there is more than one. That's why I want to keep it behind experimental.histeditng config knob for now. I think on knob for all new histedit behaviors is better because we will test all of them together and testers will need to turn it on only once to get all new nice things.