[MERGE] updated index bisection

[Robert Collins]

This is an updated bisection support for GraphIndex. It covers a corner
case I thought impossible when first writing the support. (doh).

With this, I have error free bisection support for packs; badly needed
because of the combination of all text indices into one index.

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