In prose diffs, use hash-and-diff for coarse "level 0" diffing to scale better
- On a large document (~3K top level blocks), saw a more sensible diff, instead of the whole thing falling back to "everything changed" mode.
- On a small document, still saw a sensible granular diff.
Maniphest Tasks: T13414
Differential Revision: https://secure.phabricator.com/D20839