bug: update-stack must be redeployed if new output is needed

[OlafConijn]

given a stack that contains cross-region (possibly cross-account) references. if a resource in regionB takes a new dependency on a resource in regionA, then the stack should be redeployed to regionA to ensure that the output is added to the stack in regionA. the output is needed when deploying to regionB.

currently it seems that the output is not taken into account when deciding whether or not to deploy to regionA. as a result there is no output.

workaround is to ForceDeploy the stack, forcing the output to be added to regionA.