> What happens if you make a mistake while committing - maybe forget the file, or commit wrong one?

Then you have proven that you are human.

> Fossil has no way to fix it, by design,

As an 18-year-long contributor to fossil i can assure you that this is absolutely not true. Fossil can amend any checkin and it can move checkins to other branches. Go to sqlite.org/src/timeline?r=mistake to see many examples of where human failing has been both demonstrated and accounted for without breaking anything.