Move zsh completion logic to new subcommand: diff

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -644,7 +644,9 @@ __docker_container_subcommand() {
             esac
             ;;
         (diff)
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                "($help -)*:containers:__docker_complete_containers" && ret=0
             ;;
         (exec)
             __docker_subcommand && ret=0
@@ -1896,9 +1898,7 @@ __docker_subcommand() {
             esac
             ;;
         (diff)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                "($help -)*:containers:__docker_complete_containers" && ret=0
+            __docker_container_subcommand && ret=0
             ;;
         (events)
             _arguments $(__docker_arguments) \