update on branch stacking

[Martin Pool]

Testing against Launchpad and Bazaar found a few bugs in stacking,
most of which are now fixed or in review.  Thanks to everyone who
tested and fixed them - not least Andrew, Jelmer, Ian, and Robert.

There is one more moderately serious one,
<https://bugs.launchpad.net/bzr/+bug/253891> for which I have a
failing test but not a fix and that is that you can get stuck by doing
a model-changing upgrade (such as to 1.6-rich-root) on a branch which
has branches stacked on it.  It doesn't damage anything, but we
currently give an error when you try to upgrade or open the stacked
branch.

-- 
Martin <http://launchpad.net/~mbp/>