[MERGE] KnitGraphIndex and supporting work

[Robert Collins]

This adds in one of the key things that let me get a kndx-less
repository up and running quickly - an adapter from the Knit API to the
Index API.

This works by having the knit index use an underlying GraphIndex for
reading data, and issue a callback on data insertion (which can then be
put into a temporary Index, and dumped to disk on completion).

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