Ask HN: How to commit comments to different repo?

Does anyone know of a way, via software or some other means, to be able to leave yourself comments that can be saved without getting overwritten by pulling a git repo?

I want to be able to go through an open source codebase and leave myself comments so I can understand it. But obviously, I don't own that repo and can't commit my comments to be saved. And in addition, any time I updated the code by pulling, I'd overwrite my comments or have a bunch of conflicts (assuming git doesn't throw a fit with unsaved changes in the first place).

So if there was a way to commit all my comments to one repo, then I could update the code and comment separately and have it all work out. Does anyone know of a way to do this? Even if it's a PHPStorm plugin or something IDE based, that would be fine.