Allow pushing individual branches in stack

[AaronMoat]

Dependent branches are currently intended to be pushed all at once, and to be used with stacked PRs.

I would generally prefer to push the bottom of the stack, get it PRed & merged, then move on to the next one.

Would it be possible to support pushing individual branches of a stack? Or perhaps just the bottom of the stack (as pushing dependent branches by themself probably doesn't make sense)?

[Byron]

Thanks a lot for sharing! I'd also think that this is a feature I'd want, and have a feeling that we have to stop pushing all dependent branches at some point anyway in order to give more control to the user.

CC @krlvi as I think in single-branch mode with non-workspace branches we wouldn't be able to do this anymore. From there maybe there is an avenue towards refining this behaviour in workspaces as well.