[MERGE] Fix bzr failing to stack when a server requests it and the branch it is pushing from cannot stack but the branch it should stack on can.

[Robert Collins]

As it says, this fixes bug 345169. Less sucking pushes to lp, yay.

I'm not entirely happy with the test coverage, but OTOH no tests broke
in the core when I fixed this. For now, I think its sufficient - this
whole area needs an overhaul anyhow.

-Rob
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