Package rev defines structure and syntax for specifying revisions of a dataset history. Much of this is inspired by git revisions: https://git-scm.com/docs/gitrevisions
Unlike git, Qri is aware of the underlying data model it's selecting against, so revisions can have conventional names for specifying fields of a dataset
const AllGenerations = -1
AllGenerations represents all the generations of a dataset's history
Rev names a field of a dataset at a snapshot
NewAllRevisions returns a Rev struct that represents all revisions.
ParseRev turns a string into a revision
ParseRevs turns a comma-separated list of revisions into a slice of revisions