Issue ID.
Most issue trackers have a format that they can parse issue IDs out of commit messages and group commits with issues.
If you don't use an issue tracker, you should, it doesn't matter you are a solo developer, I've elaborated on the matter on another answer:
Use an issue tracker. It doesn't matter you are a lone wolf, keep track of everything you do for your project, whether it's a feature or a bug. Make a feature / components list. Mark truly essential components as version 1.0 and all else as version 2.0. And then delete everything that's marked as 2.0.
Ok, since the question got a mod notification for longer answers, I feel obliged to expand:
Yes, I just put the issue ID in commit comments, nothing else. That's when on solo projects, on team projects it's a whole other story. I'm not advocating putting just the issue ID, there should be something more there, but:
- On solo projects it's extremely tempting to not document anything,
- Putting the issue ID means by default that you've gone through the process of setting up an issue tracker,
- That's good enough for me.
Effectively using an issue tracker when developing solo it's a far bigger achievement than commit comments. It'd be nice if I had the motivation to add something helpful in the comments, but I don't.