Replace the 'unpack' command with a more general 'get'.
'cabal get PACKAGE' is the new name of 'cabal unpack'. 'cabal get --source-repository' reads the source-repositories from a package's description, determines which VCS to use, and then creates a local repository or branch of the package's repository. 'cabal get --source-repository=[head|this|...]' additionally allows to specify which source-repository to use. Based on the code originally written by John Millikin <email@example.com>.