Your mention of commit messages resonates strongly with my own professional experiences, and I'm sure with many others' as well.

Several months ago I started working with a commercial code base that has about 3 years of commits and over a dozen contributors, but very few inline comments. Navigating and refactoring it is usually a fairly reasonable process due to well-named symbols, module organization, and test cases.

When that's not enough though, I pull up the "git blame" history in my editor, and a rich story unfolds, telling me things like about how old a block of code is, how recently that one line in the middle was changed, and why that seemingly benign extra bit of code is sitting there. Sometimes the commit messages aren't as detailed as one might prefer, but you can often still get a lot of useful insight just from the date stamps.