Provide way of seeing what commands *were* run

[jhclark]

Currently, "commands" shows what commands will be run for a particular package.

It would also be useful to see what commands (and input files, etc.) were run to produce the current output. If any dependencies have since been moved to the attic, this commands should output the attic path to the previous output, but insert a bash command, communicating the dangers of this to the user (e.g. any hardcoded absolute paths in task artifacts may break.)