[MERGE][#291046] Fix pushing an unstackable branch + stackable repo when there is a default stacking policy.

[Aaron Bentley]

Andrew Bennetts wrote:
> The correct fix I think is to behave as if the command-line had said --stacked
> --stacked-on=foo, i.e. upgrade the choice of branch format we create if stacking
> is requested.  So this patch does this.

I don't think that is the correct fix.  Default stacking policies are
supposed to be non-intrusive.  I would say the correct fix is not to
stack in that circumstance.

Aaron