Manual:$wgExternalDiffEngine

Details
Name of the external diff engine to use, or false to use the internal engine.

The possible values are:
 * false: wikidiff2>Special:MyLanguage/Extension:Wikidiff2|wikidiff2 if available, PHP implementation else.
 * wikidiff2: Wikimedia's fast difference engine implemented as a PHP/HHVM module.
 * wikidiff and wikidiff3 are treated as false for backwards compatibility.
 * any other string is treated as a path to external diff executable.

This setting replaces .