When WT4.set_parent_trees() is called, sometimes we can use
update_basis_by_delta instead of the full set_parent_trees call.
If there is a single new tree, and the repository supports fast deltas,
and ...
This makes 'bzr uncommit' when there are no merges drop from 15s down to
2s. It should also help pull and update, but still debugging why it
isn't triggered as expected.