[PATCH 0 of 3] revlog: read/cache chunks in fixed windows of 64 KB

[Brodie Rao]

This series attempts to improve the revlog chunk cache in cases where Hg
isn't iterating over the revlog in forward order. It does this by caching
chunks in windows that include data both before and after the requested data.

I also add the ability to tune the chunk cache window size (as an
undocumented internal setting).