With the 'what' of course I don't mean 'what happened', as the latter would be actually kind of the 'why' you're making the change. So with the 'what' I mean 'what is being changed', some people just write what file are changing and what they are adding to it, explaining how the code is changing in the diff, but not why the code is changing.
As for code comments, I think you're basically wrong. If you can summarize some fragment of code under a sentence, extract that piece of code into a method and name that method in the same way you were going to write that comment; otherwise comments get outdated easily. These kind of techniques are well explained in a very good book I read a while ago, it's called "Clean Code".