Introduce two new filters to the post revisions screen:

 * `process_text_diff_html` for contextually filtering a diffed line. Allows for the line to be processed in a different manner to the default `htmlspecialchars`.
 * `revision_text_diff_options` for filtering the options passed to `wp_text_diff()` when viewing a post revision.

Fixes #24908
Props adamsilverstein

Built from https://develop.svn.wordpress.org/trunk@30396


git-svn-id: http://core.svn.wordpress.org/trunk@30392 1a063a9b-81f0-0310-95a4-ce76da25c4cd