How to reproduce:
1. Create a new project and open it
2. In the Hierarchy Window create a cube and drag it into the Project Window to create a prefab
3. Open the prefab, drag its root to the Project Window
4. Once the "Do you want to create a variant" dialog shows up - cancel it
5. Observe that the prefab is dirtied

Expected result: canceling new prefab/variant creation dialog does not change/dirty the prefab in any way
Actual result: cancelling new prefab/variant creation dialog dirties the prefab

Reproduced in: 2021.1.0a2, 2020.2.0b7, 2020.1.8f1, 2019.4.12f1, 2018.4.28f1