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/helpers/pool_versions_helper.rb

@@ -21,6 +21,6 @@ module PoolVersionsHelper
 
   def pool_page_diff(pool_version, other_version)
     pattern = Regexp.new('(?:<.+?>)|(?:\w+)|(?:[ \t]+)|(?:\r?\n)|(?:.+?)')
-    DiffBuilder.new(other_version.description, pool_version.description, pattern).build
+    DiffBuilder.new(pool_version.description, other_version.description, pattern).build
   end
 end

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;')}