Stack overflow inverting parent relation

[talex5]

If you use the history view (or sync) to move from a revision where A is a parent of B to one where B is a parent of A in one step then the display code will fail with a stack overflow.

This is because it tries to show the old A fading out, but the old A now has a child B, which has a child A, ...

We should either detect the loop and stop, or refuse to add new children to things that are fading out.