Allow to resolve save conflicts using the merge editor

The built-in mechanism to handle FS conflicts nowadays still uses the diff editor: <img width="351" alt="image" src="https://user-images.githubusercontent.com/22350/181455444-1cf86beb-bc8d-4004-b1ae-773d52c3de75.png"> It would be awesome if it could instead use the merge editor (with no base). Additionally, we could rename `Compare` to `Resolve Conflicts`.