MAGETWO-66235: [GitHub][PR] Prevent overwriting grunt config #8910

 - Merge Pull Request #8910 from Igloczek/magento2:prevent-overwriting-grunt-config

Author: ishakhsuvarov

.gitignore

@@ -25,6 +25,8 @@ atlassian*
 /.grunt
 /Gruntfile.js
 /package.json
+/grunt-config.json
+/dev/tools/grunt/configs/local-themes.js
 
 /pub/media/*.*
 !/pub/media/.htaccess