[Courgette] Fix sorting in QSufSort's split().
[Courgette] Fix sorting in QSufSort's split().

BSDiff implements Quicksort in its adaptation of QSufSort. To improve
robustness, we reverted the simplified pivot selection algorithm to
the original one due to Bentley & McIlroy used in QSufSort:

http://www.larsson.dogma.net/qsufsort.c

Also reverting split() selection sort threshold from 6 back to 16.

Resulting Courgette-gen and QSufSort has same performance as before
(preliminary: ~1% faster).

BUG=605565

Review-Url: https://codereview.chromium.org/1914303002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390134}

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