If you have more than one IDLE window open (even a shell), it's not necessary to shut down.  Just close the edit window, confirm the unsaved warning, and use the Recent Files menu item to reopen with the current disk contents.  We are talking about five mouse clicks instead of three, counting confirmation dialogs. In my opinion, it's not worth complicating IDLE and confusing beginners with a 'revert' menu item to save two clicks every other day or so.

I normally use IDLE with svn and never feel the need to reload something that was merged.  After all, you should have updated before you opened the file in IDLE.  If you modified the file in IDLE, saved, and checked it in, IDLE's copy is the same as svn.

Maybe you have a use case I haven't considered.  If it's a good one, then a (normally inactive) IDLE extension might be warranted for those who really need this.

Adding a warning when saving that the file has changed on disk is a good emacs feature which has saved my bacon a number of times.  That would be a good feature to add to IDLE, but that's a different request.  Rejecting this one.