Don’t know whether it is good place for a feature request. Sorry, if it does not.
When I review a patchset, changed lines of code is not the only object of review. As a reviewer I should verify that commit messages contain all required information: motivation of the change (why it is made), clear description of the changes, explanation of non-obvious details. In some projects we even include requests for the documentation team right into commit messages.
It would be glad to be able to comment a block in a commit message within GitHub’s pull request.