Add a "summary" prose diff mode which omits unchanged text

Summary:
Ref T7643. When a user edits a large wiki page, we currently show the entire page in the diff.

Provide a mode where we summarize unchanged text. This tries to just remove the middle section of big "=" changes that have multiple lines/paragraphs.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16097