Make PhutilProseDifferenceEngine degrade on large inputs instead of consuming all RAM in the entire world

Summary:
Fixes T11743. For very large inputs which can't be simplified (e.g., dissimilar text at the beginning and end across a very large number of paragraphs) we currently try to build an edit distance matrix, but this is exponential in space and time and slowness and how PHP-ish it is.

Instead, just give up for very large inputs. This will still prose-diff any corpuses with fewer than 128 paragraphs, which is the vast majority of documents.

Test Plan:

• Created a degnerate Paste similar to the one in T11743.
• Viewed change details and ran bin/worker execute --id <id> --trace before and after patch.
  • Before: everything hung forever.
  • After: everything worked great, although the diff wasn't perfect.

Reviewers: chad

Reviewed By: chad

Subscribers: wizsrk, gregprice

Maniphest Tasks: T11743

Differential Revision: https://secure.phabricator.com/D16682