[MERGE] Create new mutable tree method update_to_one_parent_via_delta for eventual use by commit.

[Robert Collins]

This adds a method that will allow commit to give the working tree a
delta to update its basis tree to be a different tree. I plan to write a
workingtree_4 specific version of this, if the basic idea and tests look
fine. Following on from that I will have CommitBuilder output a delta
directly.

I estimate this to save 5%-10% at least during commit.

-Rob


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