Morning clean up.

Removed useless function get_changes. Moved current_diff out of this.settings (and along this.changes)
Handle the case where comparing identical files (e.g. changes.length==0).

ajax.html uses https for jquery so that it can be hosted on a https server.
ajax.html uses full available screen width, auto height is trickier (note that call to _auto_height is commented out in mergely.js).