Regression in [0] which cleared the redo-panel if an operator added
its own undo step. This worked for sculpt to fix T101743, but caused
the redo-panel to be cleared for actions who's undo steps where created
by nested operators (which is the case for the spin operator).
Fix by checking an undo-step is added without registering an operator.
[0]: f68e50a263
c1beaea80f