> Any mental model you had at week 1 is now irrelevant as the code has changed in unknown ways.

Isn't that what good git commit messages are for though? (Or any other source code versioning tool.)

I'm not saying tests aren't useful, but often times I find reading a file diff history is way more useful to understanding it than its tests.

IMO, tests should be there only to provide some level of feeling safe modifying files, as you can easily know if you messed up something.