Save changes prompt appear even once all changes were undone

**shmulikk** ([Bug 57040](https://bz.apache.org/bugzilla//show_bug.cgi?id=57040&redirect=false)):
I've started to work with the most recent nightly build (r1628112) to get first feeling with the undo/redo feature.

Once all changes were reverted / undone - closing the script still shows a dialog to save or discard changes.

Version: Nightly
Severity: normal
OS: All

Blocks:
* https://github.com/apache/jmeter/issues/1930