The "/wdiff" page:

Show the changes to a wiki page.

Query parameters:

     id=HASH           Hash prefix for the child version to be diffed.
     rid=INTEGER       RecordID for the child version
     pid=HASH          Hash prefix for the parent.

The "id" query parameter is required.  "pid" is optional.  If "pid"
is omitted, then the diff is against the first parent of the child.