Commit the paths one by one to Jekyll.sanitized_path with the proper initial base.

Prevents errors like these:

[39/78] TestPost#test_: A Post processing posts should not be writable outside of destination. /Users/parker/jekyll/jekyll/test/dest
/Users/parker/jekyll/jekyll/test/dest/Users/parker/jekyll/baddie.html
 = 0.01 s
  1) Failure:
TestPost#test_: A Post processing posts should not be writable outside of destination.  [/Users/parker/jekyll/jekyll/test/test_post.rb:152]:
Failed assertion, no message given.

lib/jekyll/site.rb

@@ -100,7 +100,9 @@ module Jekyll
     #
     # Returns a path which is prefixed with the source directory.
     def in_source_dir(*paths)
-      Jekyll.sanitized_path(source, File.join(*paths.flatten))
+      paths.reduce(source) do |base, path|
+        Jekyll.sanitized_path(base, path)
+      end
     end
 
     # Public: Prefix a given path with the destination directory.
@@ -110,7 +112,9 @@ module Jekyll
     #
     # Returns a path which is prefixed with the destination directory.
     def in_dest_dir(*paths)
-      Jekyll.sanitized_path(dest, File.join(*paths.flatten))
+      paths.reduce(dest) do |base, path|
+        Jekyll.sanitized_path(base, path)
+      end
     end
 
     # The list of collections and their corresponding Jekyll::Collection instances.