When changing a saved file, an appears next to the file-name, indicating that the file has changed. Undoing this change removes the asterisk, since the text in the editor now reflects the text in the file.
The same logic could also apply to redo - priority really low but might be easy to implement 
- Save a file
- Type one character (asterisk appears)
- Save again (asterisk disappears)
4 Type another character (asterisk appears) - Undo (asterisk disappears)
- Undo again(asterisk appears)
- Redo: Asterisk stays even though the editor content now matches with the saved file
- Redo and Undo again: Asterisk disappears as expected