Rethink diff limits

Rethink diff limits

Now, instead of "if your changed paragraphs are larger than 10Kb, you're
screwed":
* Instead of relying on overall length, estimate complexity after splitting to words
  and taking any equal head and tail out of equation.
* Estimate based on words changed, which better reflects the actual complexity
  of generating a diff.
* New limit is determined scientifically, i.e. "above that number XDebug starts
  complaining about recursion limits reached in Vagrant".

Caveat: if new limits are hit, the consequences are more widespread as all adjacent
changed paragraphs are displayed without word level diffs, as opposed to only the
paragraph that's too long being affected. However, the new limit is much higher and
in wikitext you're supposed to put empty lines between paragraphs anyway, negating
this problem.

Bug: T128697
Change-Id: I4e91c7c40f5afdd116b847a859b8517522302489