Preventing Jekyll from picking up the output directory as a source

If you attempt to have Jekyll output to a subdirectory of the
source it may recursively copy the destination as it writes it.

lib/jekyll/site.rb

@@ -86,6 +86,7 @@ module Jekyll
 
       entries.each do |f|
         if File.directory?(File.join(base, f))
+          next if self.dest.sub(/\/$/, '') == File.join(base, f)
           transform_pages(File.join(dir, f))
         else
           first3 = File.open(File.join(self.source, dir, f)) { |fd| fd.read(3) }