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

[Andrew Bennetts]

Andrew Bennetts wrote:
> Hi,
> 
> This patch fixes a bug when pushing a new stacked branch.  The bug causes a push

Oh, and this patch also fixes TestCaseWithBranch's make_branch_builder to make a
branch in the right format, not the default!

-Andrew.