Up to now 'upgrade' took no args and tried to upgrade all installed
packages to the latest versions. It retains that mode but also has
a new mode rather like 'cabal install'. The difference is that with
$ cabal install foo
it means install latest version of foo but otherwise prefer the
installed versions of deps of foo, while
$ cabal upgrade foo
means install the latest version of foo and also the latest
versions of all the dependencies of foo.