update on stack merging

[Martin Pool]

I've tracked this down a bit more but unfortunately not fixed it yet.

It looks like the problem at the moment is that there are two objects
inside the repository that know about stacking: there are
KnitVersionedFiles and also RepositoryPackCollections which contain an
AggregateIndex.  When repo.add_fallback_repository is called it
inserts the fallback repository into the first but not the second.
Both of them provide a get_parent_map call, so you can get
surprisingly different results.

What I'm going to try next is to have
KnitPackRepository.add_fallback_repository poke it into the pack
collection too, which should at least make sure they give consistent
results.  There is no obvious api for that to add at the moment so I
have some doubt as to whether this is the way Robert intended it would
work or not.  It seems a bit less than ideal that they'd both need to
be configured this way rather than one of them being in charge of
stacking and it just being configured in one place.

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