I want to associate custom metadata with a `git commit`. Specifically to record a review ID from a code review but it could be anything. Tags seem a natural way to do that but I expect to have a review for every commit and I don't want to clutter `gitk` with tons of tags. Is there some other mechanism to add custom metadata? Can I make certain tags invisible? If I could tell `gitk` not to display tags matching some pattern or RE, that would likely work but I don't see a way to do that.