Comment Git commits

Do you know any web app (OSS, SaaS, self-hosted) which can be integrated in self-hosted Git server for providing functionality like custom comments on commited code itself? Workflow should be - I open Git history, browse a code for each commit, choose a file, place a comment in any line. Basically like a discussion with collaborators on that project, but related to the code. There should be also email notifications and maybe some other basic features for that kind of collaboration. So, doing-one-thing-well kind of app.