I want to make a generic Resource object representing something with
versioned contents, like an S3 object or a git repo. For each Resource
object subclass, there would be State, StateId, DiffState, DiffMetadata
and Diff subclasses. So an S3 object's StateId class would follow
the AWS naming convention for S3 versioning, and the Git StateId class
would be a commit reference.
But there's more than one way to access a resource. For instance, you
can ask Github about the state of a git repo or you can shell out to git
on your local machine. Even though the mechanics of 'shell out to git'