Add --dry-run for add/update etc.

[IanButterworth]

We previously had preview mode, which was removed and replaced with a more robust undo/redo, but I think there's still benefit of having something like preview but instead utilize the improved undo mechanics automatically at the end of the operation.

i.e. if undo is robust, then this should be.

I also thought --dry-run was more typical, given that's what cargo has.

[IanButterworth]

This needs a bit more thought because undo states are only set if something changes, so it could currently undo too far back.