[MERGE] StackableBranch

[Ian Clatworthy]

This patch adds stackable branches. You guessed it - it's
part of the Stackable Branches work by Robert. :-)
Actually, it's arguably the key patch in the series in that
the ones above this one are largely UI and pretty straight
forward.

I've reviewed this and made several tweaks. A diff covering
many, but not all of my tweaks, is attached. The other tweaks
I made were done earlier during the bzr.dev merge, namely:

* fixed LockableFile deprecation warnings/issues

* replaced some of the super(BzrBranch6, ...) calls with
  super(BzrBranch7, ...) after BzrBranch6 was renamed to
  BzrBranch7

* fixed the search_missing_revision_ids code to check ghost
  vs non-ghost and throw NoSuchRevision when required by the
  API

* added "shallow_branch_url = None" to an except clause to
  fix a broken test

* corrected the version # when the new branch formats were
  made available in bzr.dev (assuming this makes it into 1.6
  of course :-) ).

Here are the outstanding issues/questions I'd like feedback
from Robert (and others if interested) on:

1. BzrBranch7._get_fallback_repository(url) requires that
   the url be a branch url, i.e. a url of a shared repo
   without a branch looks like it wil fail. Is that by
   design? Acceptable for now, or an oversight?

2. Is my algorithm in InterPackRepo.search_missing_revision_ids
   ok or too complex? The comment says "Find ghosts:" so I'm
   wondering whether the "Double query here:" bit of code can
   be removed if I just collect ghosts and skip the non ghosts?

Ian C.
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