dburrows/ projects/ aptitude/ todo/ entry/ Null solution handling

The null solution -- that is, the one that reverts all the user's changes -- should be handled more gracefully. Right now it's silently thrown away whenever it appears, which is confusing if it turns out to actually be what you want. I think that at least one of the following approaches should be taken instead:

Also, I need a better name than null solution, since that makes me think of the empty solution. Maybe call it the keep everything solution? :-)