Fix autocompletion of nixops machine names

Machine names were being completed from _known_hosts. These commonly
don't align with actual machine names in a nixops deployment.

This commit introduces a new completion type for nixops machines based
on output from 'nixops info'. Furthermore, if the user had already
supplied a --state or --deployment argument, then these are respected.

_nix

@@ -872,7 +872,7 @@ function _nix_completion () {
                '(--deployment|-d)'{--deployment,-d}':->nixops_deployments'
                '--confirm' '--debug')
            local -a nixops_include_exclude=(
-               '--include:_known_hosts' '--exclude:_known_hosts')
+               '--include:->nixops_machines' '--exclude:->nixops_machines')
            local -a nixops_search_path_args=('-I:->option-INCLUDE')
 
 
@@ -948,7 +948,7 @@ function _nix_completion () {
                        ;;
                    show-option)
                        main_options=(
-                           '--xml' ':_known_hosts' ':->nixops_option')
+                           '--xml' ':->nixops_machines' ':->nixops_option')
                        break
                        ;;
                    set-args)
@@ -959,7 +959,7 @@ function _nix_completion () {
                        break
                        ;;
                    show-console-output)
-                       main_options=(':_known_hosts')
+                       main_options=(':->nixops_machines')
                        break
                        ;;
                    export)
@@ -986,6 +986,12 @@ function _nix_completion () {
                 fi
             fi
 
+            local deploymentcmd
+            if [[ "${opts[--deployment]}" || "${opts[-d]}" ]]; then
+                local deployment="${opt_args[--deployment]:-${opt_args[-d]}}"
+                deploymentcmd="--deployment $deployment"
+            fi
+
             case "$state" in
                 nixops_deployments)
                     local -a deployments=()
@@ -994,6 +1000,13 @@ function _nix_completion () {
                     COMPREPLY=($(compgen -W "${deployments[*]}" -- "$cur"))
                     return
                     ;;
+                nixops_machines)
+                    local -a machines=()
+                    while read -r _ name _; do machines+=("$name")
+                    done < <(nixops info $statecmd $deploymentcmd | tail -n +10 | head -n -1)
+                    COMPREPLY=($(compgen -W "${machines[*]}" -- "$cur"))
+                    return
+                    ;;
             esac
            ;;
         nix)