gh-135551: Change how sorting picks minimum run length (#135553)

gh-135551: Change how sorting picks minimum run length (#135553)

New scheme from Stefan Pochmann for picking minimum run lengths.

By allowing them to change a little from one run to the next, it's possible to
arrange for that all merges, at all levels, strongly tend to be as evenly balanced
as possible, for randomly ordered data. Meaning the number of initial runs is a
power of 2, and all merges involve runs whose lengths differ by no more than 1.