CountHook is an implementation of terraform.Hook which is used to
calculate how many resources were added, changed, or destroyed during an
apply. This hook was previously injected in the local backend code,
which means that the apply command code has no access to these counts.
This commit moves the CountHook code into the command package, and
removes an unused instance of the hook in the plan code path. The goal
here is moving UI code into the command package.
Looking through the operations in node_resource_apply and
node_resource_destroy, there are multiple nil checks for diffApply, so
one we need to continue to assume that the diff can be nil through there
until we ensure that it is non-nill in all cases.
So regardless of how we came to get a nil diff in the UiHook PreApply
method, we need to check it.
Alisdair McDiarmid