This PR adds a new `pulumi history` command, which prints the update history for a stack.
The local backend stores the update history in a JSON file on disk, next to the checkpoint file. The cloud backend simply provides the update metadata, and expects to receive all the data from a (NYI) `/history` REST endpoint.
`pkg/backend/updates.go` defines the data that is being persisted. The way the data is wired through the system is adding a new `backend.UpdateMetadata` parameter to a Stack/Backend's `Update` and `Destroy` methods.
I use `tests/integration/stack_outputs/` as the simple app for the related tests, hence the addition to the `.gitignore` and fixing the name in the `Pulumi.yaml`.
Fixes#636.
4c217fd358