dirstate basis updates require working tree to contain entries

[Robert Collins]

This seems bogus to me; it seems reasonable for pull to update the basis
tree before updating the working tree, and to use the delta based
update. It's no different to calling set_parent_ids, at least in
principle it should behave the same and have the same results.

This is interfering a tad with delta application tests, but not
insurmountably, so I'm simply filing a bug for now.

 affects bzr
 status triaged
 importance medium