Feature Request: Add comments to commits

Git commit messages quality is very important when you investigate your history. Often it would be useful to be able to attach comments to commits messages for instance to add forgotten information. Those comments should be shown when `git show`ing a commit. Could be implemented as commits that use a new field to link to the commit they should be shown with, along with some index to make it performant.

Services like Github that already have comments on commits and PRs, could integrate with this to make comments made on the platform part of git.