More fix to 'topic/post edited' tipsy link

* Use the buildin function to check if there is content change

* Add missing tipsy content to post template

* The post diff should be based on getPrevRevisionId() than
  getLastContentEditId(), which reverts 105421

Change-Id: I42dcfe8d3014470b0383e2d5680fadb22fcb5150
Merge "More fix to 'topic/post edited' tipsy link"