Problem: No convenient way to programmatically get the revision that
would be checked out after `vim.pack.update()` (with `offline=true`).
Doing this manually requires resolving `spec.version` which is not
trivial. This data can be useful for custom reporting of pending
updates or third party confirmation step.
Solution: Make `get()` include a new field for the revision that points
at the state after applying pending update. This is also the same as
the revision of resolved `spec.version`.
b9c4329c35