New command is now `kubectl diff` rather than `kubectl alpha diff` since
it's moving out of alpha soon, and will be using dry-run apply to
produce the diff rather than the custom merge logic.
`kubectl alpha diff` lets you diff your resources against live
resources, or last applied, or even preview what changes are going to be
applied on the cluster.
This is still quite premature, and mostly untested.
Alpha commands can be added under `kubectl alpha` and are always
accessible (regardless of feature gates). If no alpha commands have been
defined then `alpha` is not displayed in `help`.
