Just copy this code to the bookmark pane of your browser. Whenever you are creating a PR in github, just press the bookmarklet and voila, All the commit messages are now aggregated in your PR comment box.
The code:
javascript:(function(){
msgs = document.getElementsByClassName("commit-message");
commitMsgs = "";
for (i=0; i<msgs.length; i++) {
commitMsgs += msgs[i].getElementsByClassName("message")[0].getAttribute("title") + "\n\n";
}
document.getElementById("pull_request_body").value = commitMsgs;
})()