[WIP] lazy revlog index parsing

[Bryan O'Sullivan]

The current index parser is eager, and does a lot of unnecessary work on
large revlogs.

This causes e.g. "hg tip" to take 0.3 seconds on a repo that contains
300,000 commits.

I've adopted a patch that Matt started on, which parses the index on
demand. This brings the performance of "hg tip" in the large-commit case
back to instantaneous.

https://gist.github.com/2039952
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