[MERGE] Clean up push, fixes bug #259275

[Aaron Bentley]

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Hi all,

This patch updates the push code to allow automatic upgrades when
- --stacked is specified.  In the process, there's a lot of clean up.

I found that BzrDirPreSplitOut.clone isn't being used anywhere, because
I accidentally broke it, and it passed the test suite.  So I removed it,
causing such classes to use the generic clone implementation.

All BzrDirs share a common clone_on_transport implementation already,
and this is the main way cloning is performed.  By adding a stacked_on
parameter to clone_on_transport, we are able to reuse the format upgrade
code in cloning_metadir().

Because of the bizarre irregularities in the PreSplitOut formats, I had
to update the WorkingTree so that create_tree actually does create a tree.

With this patch, branch stacking policies should be fully functional,
without unduly upgrading formats.

I still feel sprout and clone need a major overhaul, though.  There's
still a lot of duplicate or awkward code.

Aaron
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