memoize helper methods in site-cleaner (#6808)

Merge pull request 6808

lib/jekyll/cleaner.rb

@@ -57,9 +57,9 @@ module Jekyll
     #
     # Returns a Set with the file paths
     def new_files
-      files = Set.new
-      site.each_site_file { |item| files << item.destination(site.dest) }
-      files
+      @new_files ||= Set.new.tap do |files|
+        site.each_site_file { |item| files << item.destination(site.dest) }
+      end
     end
 
     # Private: The list of directories to be created when site is built.
@@ -67,7 +67,7 @@ module Jekyll
     #
     # Returns a Set with the directory paths
     def new_dirs
-      new_files.map { |file| parent_dirs(file) }.flatten.to_set
+      @new_dirs ||= new_files.map { |file| parent_dirs(file) }.flatten.to_set
     end
 
     # Private: The list of parent directories of a given file

lib/jekyll/site.rb

@@ -97,6 +97,7 @@ module Jekyll
       @docs_to_write = nil
       @regenerator.clear_cache
       @liquid_renderer.reset
+      @site_cleaner = nil
 
       if limit_posts < 0
         raise ArgumentError, "limit_posts must be a non-negative number"