0.92 semi-freeze, but still waiting for packs

[Martin Pool]

The pack repository format led by Robert is showing great results:
reported by independent users on irc as faster than hg for some
operations and not much slower for others.   This is pretty exciting.
Now we just need to land it.

There are two more things to do to get it in:
 - https://bugs.launchpad.net/bzr/+bug/155730 - fix reconcile to avoid
problems transferring old bzr revisions between knit and pack
repositories - spiv is working on this now

 - do all the review cleanups suggested for packs - i am doing this now

 - https://bugs.edge.launchpad.net/bzr/+bug/156091 - fix packs to copy
all necessary data even when reconcile has not been run on the origin
- this is arguably unnecessary, but I think it'd be much nicer to have
the repository just work without needing extra preparation, and it
looks like it can be done at a low cost.  if anyone else would like to
take a look, that would be great.

For now, please continue landing bug or performance fixes, but ease
off on new features so we can release 0.92rc1 when packs are in.

-- 
Martin