[MERGE][1.13] Fix bug when pushing stackable branch in unstackable repo to default-stacking target.

[Andrew Bennetts]

I'm not sure if this is a regression or not, but the fix is very simple so it
would be nice to have in 1.13.

If you have a stackable-format branch in an unstackable repo (e.g.
BzrBranchFormat7 in RepositoryFormatKnitPack1), and push it to a location with a
default stacking policy, it will fail with:

    bzr: ERROR: The repository ... is not a stackable format. You will need to
    upgrade the repository to permit branch stacking.

This patch fixes that.

-Andrew.

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