One level of rollback is enough

We currently support one level of rollback history. This is rather limited, but simplified our CLI in the early going. This article captures some thoughts around extending rollback history.

When is rollback important?

• Undo an update/switch
• Revert to a previous set of dependencies versions (might be more than one step)

We could dig into named histories (e.g. "zero resolve <name>") and timestamped histories, along with the counterpart rollback support ("zero rollback <name>"), but resolve.history seems like only part of the problem when dealing with longer intervals of change. For example, code and configuration (including ivy.xml) is more likely to change over longer intervals -- so reverting resolve.history alone is not very helpful.