Improve adding a new branch to a stack when existing branches are already merged

[geofflamrock]

When you add a new branch to a stack, it creates the new branch from the bottom of the stack e.g. the last branch. This is normally fine, however if the last branch has already been merged it is likely not to be up to date with the head of the stack (probably main) because merged branches get skipped in stack update. This means it'll start behind already, and could have conflicts. It would probably be better in this scenario to create the new branch from the source branch. The general approach I think it "create the new branch from the lowest branch in the stack that isn't merged". Should probably also run a stack update before creating the new branch to make sure everything is up to date, to create a good experience.