alt Hacker NewsI thought I agreed with you at first but I'm not sure. Either we disagree on how important what and why are, or on how "why" is the defined or expressed.
I think commit messages should actually have a concise "what" in them.
I frequently enough end up looking at git log trying to sort out what changed (to track down a bug or regression), and based on the commit message, do a git show to see what the actual diffs are.
So in that context, at least, knowing what changed in a commit is actually quite useful, and why is arguably less so.
I suspect my idea of "what" and your idea of "why" overlap in this scenario.
Edit: and after typing all that, I realized your comment doesn't imply there shouldn't be a "what" described anyway so maybe I'm just discussing nothing at all.
Sure "top-line" of the message (the subject line of the email) should be concisely "what" changed, but the rest of the message (the body of the email) should be the details of "why" and "how". More details on the "what changed" is often redundant because by that point you are seeing the diff itself, but the "why" and "how" is often the real important part to a commit message.