sparse-revlog: put the native implementation of slicechunktodensity to use
When possible, the C implementation of delta chain slicing will be used.
providing a large boost in performance for this operation.
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 0.660 ms to
0.098 ms;
The full series move delta chain slicing from 1.120 ms to 0.098 ms;
Implementing _slicechunktosize into C would yield further improvements.
However, the performance seems good enough for now.
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