Cache the list of documents to be written (#6741)

Merge pull request 6741

lib/jekyll/site.rb

@@ -94,6 +94,7 @@ module Jekyll
       self.static_files = []
       self.data = {}
       @collections = nil
+      @docs_to_write = nil
       @regenerator.clear_cache
       @liquid_renderer.reset
 
@@ -311,7 +312,7 @@ module Jekyll
     #
     # Returns an Array of Documents which should be written
     def docs_to_write
-      documents.select(&:write?)
+      @docs_to_write ||= documents.select(&:write?)
     end
 
     # Get all the documents