[MERGE] set parent trees api.

[Robert Collins]

This is an implementation of the parent tree setting api I discussed
here and specced on
https://launchpad.net/products/bzr/+spec/workingtree-parent-api

Seeking a +1.

Following this being merged, I have patches to convert the callers and
deprecate the legacy functions. 

The desired optimisations should be cleanly possible with this being
present though. [though this implementation does not try to do that].

-Rob
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