Memoize the result of gsubbing @virtual_path

This gets called many times for each virtual_path, creating a new string
each time that `translate` is called. We can memoize this so that it
only happens once per virtual_path instead.

actionview/lib/action_view/helpers/translation_helper.rb

@@ -122,9 +122,12 @@ def localize(*args)
 
       private
         def scope_key_by_partial(key)
-          if key.to_s.first == "."
+          stringified_key = key.to_s
+          if stringified_key.first == "."
             if @virtual_path
-              @virtual_path.gsub(%r{/_?}, ".") + key.to_s
+              @_scope_key_by_partial_cache ||= {}
+              @_scope_key_by_partial_cache[@virtual_path] ||= @virtual_path.gsub(%r{/_?}, ".")
+              "#{@_scope_key_by_partial_cache[@virtual_path]}#{stringified_key}"
             else
               raise "Cannot use t(#{key.inspect}) shortcut because path is not available"
             end