At my first internship, many years ago, my supervisor suggested that I keep a work log. That habit has stuck with me since then, and it's proven to be extremely valuable. Some of the value comes from having date-stamped notes on previous work; I frequently search through my work log to find notes on problems and tasks.
Using commit messages helps with this sort of thing. The commit messages are not just, "did this. did that." They include reminders about why I am doing this project, the goal, concurrent efforts, etc. Git commit messages tend to prompt entries which wouldn't make it into an app called Day One on my Mac which is set up to auto-remind, whose entries tend to be about ephemeral things, or family things, not really work.
I certainly agree that detailed git commit messages are important, but I don't think I'd be able to accomplish the same thing using them instead of a work log. My log includes details of discussions with teammates, outlines of failed attempts to solve a problem, discussion of tasks that aren't code-specific, etc.
I used to keep a journal every day. Do you keep one?