The diff shows you what changed. The commit message is a comment on the diff and absolutely should say what it's for.

This enables you to easily compile release notes, for example.

Yes, it should say what it's for, but it shouldn't need to explain itself any further. Sorry if I didn't make that clear. For example, if someone refactored something, the commit log can simply say "Factorize foo", it need not explain why they were refactoring in the first place.

Comments tell why the code does what it does. Commit messages tell why the code does something different than before.

Example comment: i++; // because server starts counting at 0 (not 1)

Example commit: Our customer switched to bar (from foo), which starts counting at 0 instead

That works as long as you're adding code. But what about when you're deleting code? I absolutely want to know why you've removed something.