[MERGE] _generate_text_key_index

[Robert Collins]

This is the LRU cache enabled _generate_text_key_index. It performs,
IMNSHO, reasonably well.

I'm now working on actual reconcile integration, at which point I'll be
able to compare the two codebases and ensure that this is actually a
performance improvement.

I'd like to get this review though, even if not merged until the group
is completed.

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