[MERGE] knit._PackAccess

[Robert Collins]

This is the final 'core' change needed to allow a pack based repository
to be built on top of the rest of bzrlib without reinventing delta
composition etc etc.

* In ``bzrlib.knit`` the internal interface has been altered to use
  3-tuples (index, pos, length) rather than two-tuples (pos, length) to
  describe where data in a knit is, allowing knits to be split into
  many files. (Robert Collins)

* ``bzrlib.knit._KnitData`` split into cache management and physical
  access with two access classes - ``_PackAccess`` and ``_KnitAccess``
  defined. The former provides access into a .pack file, and the latter
  provides the current production repository form of .knit files.
  (Robert Collins)

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