[merge][rfc] branch --stacked should set up to use a stackable format

[Andrew Bennetts]

Andrew Bennetts wrote:
> Andrew Bennetts has voted comment.
[...]
> If I can stop worrying about that, this will be bb:approve :)

bb:approve

I've decided I'm ok with this.  I do fear what these methods will look like when
we add another format-feature into the code paths... but we can worry about that
when it happens rather than now.

-Andrew.