Add completion for terraform worskpace.
The terraform env command is deprecated.
The terraform workspace command should be used instead.
"terraform env" will be removed in a future Terraform version.
* Basic Terraform completion supporting all commands
* Option completion for Terraform commands
* Search command line in reverse order
* CHANGELOG entry
* Fix `terraform untaint` completion
* Use common completion functions to handle subcommands
* Use imperative form and remove CHANGELOG changes
