cabal updatelog

Have you ever run `cabal update` and then gone 'Oh shit, I can't find dependency plans anymore'. With sufficient work, you can probably get it working again, but if you need a build ASAP, you'd probably much rather roll back to the old version of cabal update. With `--index-state`, you can indeed rollback... but to what?

`cabal updatelog` to the rescue. It will tell you a list of all the most recent updates, and the flag value you can pass to set to the old one. Additionally, it would be good sense for `cabal update` to say, "Hey, to reset back to the previous state, run X". And why not also have cabal update support index-state, to rewind the global index state to some point?

CC @hvr