I also have this problem, so I made a tool that lets you write your commit messages in-advance. It helps me to focus on one problem at a time.

https://github.com/synek/git-plan

One feature I wanted to add was for it to parse your source code for comments with a specific format (e.g. `# git-plan feat xyz` or `# git-plan fix xyz`) and then stitch all the hunks together into commits for you. So all you'd have to do is comment your code and then run `git plan commit` and it would generate commits for you to confirm with y/n.

I built a prototype of that here:

https://github.com/synek/codeline

I haven't worked these for a while unfortunately :(