The "backendConfigNeedsMigration" helper evaluates the backend
configuration inline to compare it with the object previously saved in the
.terraform/terraform.tfstate file.
However, this wasn't updated to use the new "static eval" functionality
and so was treating any references to variables or function calls as
invalid, causing a spurious "backend configuration changed" error when
re-initializing the working directory with identical backend configuration
settings.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
