Altering a previous move deletes move tree

[gthazmatt]

This is a bug I introduced with my changes in 2.5.5. Steps to reproduce:

1. Play a few moves
2. Switch to review mode
3. Go back to a previous move
4. Switch to play mode
5. Click on previous move and place it somewhere else
6. Switch back to review mode

Since the move was merely repositioned, it should still have the moves that were played after it. I think I must have changed this operation to be an undo without keeping the variation and then create a new move which would have no parent.