I think that this is a great idea and has been on my mind to add in. I'm glad you thought of it too. That means it's probably a valid feature to add. 

I think the shortstat will accomplish this but this line is a bit off. We would not want to always run the diff against master. Say you are doing a production deploy, you'd want to know if there are migrations between staging and production. #match_tag_to should handle that with no issues.