I would go for the combination: first what, then why. Links break, so a link to why is not good enough when it comes to long-lived code. A good commit message

I would go for the combination: first what, then why. Links break, so a link to why is not good enough when it comes to long-lived code. A good commit message should however start by completing the sentence "when committed this will ...". This makes reading the one-line summaries of the git log the clearest to interpret what happened.