Manual:$wgExternalDiffEngine/en

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

The possible value are:
 * wikidiff: Use external C++ wikidiff module from the extensions package.
 * wikidiff2: Use external C++ wikidiff2 module from the extensions package.
 * wikidiff3: Use internal PHP wikidiff3 class.
 * other value: Use this value as the command line that generates the diff result.

This setting replaces $wgUseExternalDiffEngine.