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


git-svn-id: https://develop.svn.wordpress.org/trunk@30396 602fd350-edb4-49c9-b593-d223f7449a82