[MERGE] Add BTree based repository format flavours

[Robert Collins]

This generalises packrepo a little to allow a btree based index to be
used; I also removed the no-longer needed development1 formats (users of
those can upgrade with bzr 1.6 or 1.7 to a stable format).

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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