Fix rendering of previous values.

[toirl]

Rendering of previous values is rarly used (I only know only one old ringo based application which uses this feature)

Previous values are currently provided on rendertime. As same as for the current values this is deprectated. Values should be provided on form initialisation.

With dc26318 I disabled rendering of the diff as rendering was broken anyway.

[toirl]

If no (say None) previous values are provided the rendering of the diff should be disabled.