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

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> ^- you have some extra whitespace here.

Fixed.

> I'm a bit concerned about automatically changing the repository format.
> Specifically, if I have a 1.9 format repository, and I accidentally have
> a Branch6 branch, (because 'bzr upgrade repo' doesn't upgrading all of
> my branches), when I push this, it will actually downgrade the target to
> a 1.6 format repository.
> 
> 
> As *I* am planning to upgrade to 1.9 and start dogfooding launchpad as
> soon as it supports it, I have the feeling this is going to explicitly
> happen for me several times.
> 
> So I'd like a test dealing with a KnitPack6 format repository that shows
> that it preserves the repository format as long as it supports stacking
> already.

I've added the test, and it passes with no code changes.  It works already
because by the time require_stacking() is called in this case the new
repository has already been made (preserving the source format), so the
repository format override doesn't matter.

-Andrew.