Fixes T5041. Pretty sure this is the issue: if a diff contains a large number of identical lines longer than 30 characters, we end up paying O(N^2) for each set.

Instead, when N > 16, opt to pay 0.

Added a test which dropped from ~100s to ~0 after changes (this diff includes a reduced-strenght version of the test, since parsing a 4,000 line diff is a little bit pricey).