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.