Ensure label correct when undoing/redoing

The undo/redoEntries were already off the stack so consulting the label
while in the middle of undoing or redoing was incorrect.
Preserve undo label as basis for redoLabel

This can always be customized if necessary but in doing so an author
should consider whether the label is for an undo or redo operation
lest the label appear reversed.

This change just does that by default.