Diff viewer shows difference between draft and live

Reproduction path:

1. Open existing document in content perspective
2. Edit document
3. Change a (text) field, but do not save
4. Switch perspective, e.g. channels and return to content perspective
5. Close document and discard changes
6. Open document and see discarded changes in diff viewer
7. Edit document and see discarded changes are properly removed

Expected to see no differences in diff viewer when opening the document after discarding changes.

The behaviour of the diff viewer has been discussed internally and we concluded that it should no longer show the differences between draft and live, but between unpublished and live.