Remove duplication when generating list of entries to process

Posts and Drafts share the same logic to get the list of files to
process into objects when generating the site. Factor this logic into
its own method and use it when reading posts and reading drafts.

lib/jekyll/site.rb

@@ -187,9 +187,7 @@ module Jekyll
     #
     # Returns nothing.
     def read_posts(dir)
-      base = File.join(self.source, dir, '_posts')
-      return unless File.exists?(base)
-      entries = Dir.chdir(base) { filter_entries(Dir['**/*']) }
+      entries = get_entries(dir, '_posts')
 
       # first pass processes, but does not yet render post content
       entries.each do |f|
@@ -212,9 +210,7 @@ module Jekyll
     #
     # Returns nothing.
     def read_drafts(dir)
-      base = File.join(self.source, dir, '_drafts')
-      return unless File.exists?(base)
-      entries = Dir.chdir(base) { filter_entries(Dir['**/*']) }
+      entries = get_entries(dir, '_drafts')
 
       # first pass processes, but does not yet render draft content
       entries.each do |f|
@@ -395,5 +391,17 @@ module Jekyll
         raise "Converter implementation not found for #{klass}"
       end
     end
+
+    # Read the entries from a particular directory for processing
+    #
+    # dir - The String relative path of the directory to read
+    # subfolder - The String directory to read
+    #
+    # Returns the list of entries to process
+    def get_entries(dir, subfolder)
+      base = File.join(self.source, dir, subfolder)
+      return [] unless File.exists?(base)
+      entries = Dir.chdir(base) { filter_entries(Dir['**/*']) }
+    end
   end
 end