Fix #4263: Wiki page differences use the wrong order.

Standardize DiffBuilder to treat the first argument as the new item and
the second argument as the old item. This is the order used in most
other places.

app/logical/diff_builder.rb

@@ -12,7 +12,7 @@ class DiffBuilder
     otharr = that_text.scan(pattern)
 
     cbo = Diff::LCS::ContextDiffCallbacks.new
-    diffs = thisarr.diff(otharr, cbo)
+    diffs = otharr.diff(thisarr, cbo)
 
     escape_html = ->(str) {str.gsub(/&/, '&amp;').gsub(/</, '&lt;').gsub(/>/, '&gt;')}