The current set_clean usage is racy (the file might be modified between its
restoration and the set_clean call).
So we simply leave the file as ambiguous and the next status will fix that.
We still have to make sure the copy information is dropped, so we teach dirstate
how to do that.
The win32txt extension is confused after this because current logic is broken in
more location. However this series will ultimately fix that so we "ignore" it
for now. Fixing it now is complicated without some extra fix landing later.