brisbane:DiskFormat changes

[John Arbash Meinel]

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Attached is the last little bit of disk format changes that I have in
mind to land before we merge brisbane into bzr.dev. There is more that
we'll probably want in the end (subtrees and a different chk index),
however this is good enough for now. It just removes a couple of pieces
of cruft that I knew about and have wanted to get rid of.

In my quick tests it actually can shrink the inventory data by another
15% or so. (It remove ~4 bytes per record, so it depends on how big the
average record is.)

It also finishes off the "LABEL" discussion, by removing support for
labels. If we go with the proposed CHK index, we'll need some sort of
label back, but it isn't going to be the same as what we have now anyway.

When I thought about it, Revisions have their revision_id anyway, as do
the 'inventory' pages. CHK pages are naturally self describing. The only
thing left was text keys. I would sort of like to get the actual content
into CHK, and then just have some structure around for maintaining a per
file graph.

Also, if we don't push the graph information down into the label, then
the index can't be regenerated from the content anyway. And at least for
this release, that requires a bit more work to push the graph info down
another level. (And the bloat of compressed content, without a whole lot
of gain.)

I didn't want to just land this before other people knew about it, since
it is a disk format bump. Though hopefully the last one for a little
while at least. (It also bumps the repo format strings, etc.)

John
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