CONTRIBUTING

sparse-revlog: rework the way we enforce chunk size limit

We move from a O(N) algorithm to a O(log(N)) algorithm.

The previous algorithm was traversing the whole delta chain, looking for the
exact point where it became too big. This would result in most of the delta
chain to be traversed.

Instead, we now use a "binary" approach, slicing the chain in two until we
have a chunk of the appropriate size.

We still keep the previous algorithm for the snapshots part. There are few of
them and they are large bits of data distant from each other. So the previous
algorithm should work well in that case.

To take a practical example of restoring manifest revision '59547c40bc4c' for
a reference NetBeans repository (using sparse-revlog). The media time of the
step `slice-sparse-chain` of `perfrevlogrevision` improve from 1.109 ms to
0.660 ms.

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https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

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273ce12ad8f1` will report some style violations on a very old commit).