gh-119105: difflib: improve recursion for degenerate cases (#119131)

Code from https://github.com/pulkin, in PR
https://github.com/python/cpython/pull/119131

Greatly speeds `Differ` when there are many identically scoring pairs, by splitting the recursion near the inputs' midpoints instead of degenerating (as now) into just peeling off the first two lines.

Co-authored-by: Tim Peters <tim.peters@gmail.com>