Should use safeRestart, not immediate restart, when -restart option is given.

core/src/main/java/hudson/cli/InstallPluginCommand.java

@@ -141,7 +141,7 @@ public class InstallPluginCommand extends CLICommand {
         }
 
         if (restart)
-            h.restart();
+            h.safeRestart();
         return 0; // all success
     }