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

[Martin Pool]

In bzr.dev at the moment if you try to branch from a non-stackable
branch or repository format (ie almost any existing branch) you get an
error that stacking is not supported.  This is because by default we
preserve the format when branching.

If you give --stacked it would make sense for bzr to create a
repository and branch which can support branching.

This patch does that, though not in a totally satisfactory way.

 * adds BranchFormat.supports_stacking so we can find out about it
without needing to have a particular branch in mind
 * BzrDir.sprout forces a particular branch format if necessary
 * RepositoryPolicy similarly

Hardcoding the particular formats we want to use is not great, but
what bothers me more is getting the right place to put this policy in.
 Maybe it should be in the RepositoryPolicy objects.

-- 
Martin <http://launchpad.net/~mbp/>
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