It's surprising to me that GitHub currently doesn't attach review comments to specific commits, but only to timestamps, and I agree that it would be great to improve that. But I don't understand how a contributor can feel pressure to address all of the reviewers' comments in a single commit: They can commit as many times as they want, and only push when they feel it's ready. And in the case where they may want to off…
It is possible to comment on commits in github (you can do it by clicking on the sha1 of the commit and then making a comment on a line in the diff). But the comment won't show up in the main PR diff window.
> But I don't understand how a contributor can feel pressure to address all of the reviewers' comments in a single commit: They can commit as many times as they want, and only push when they feel it's ready.
Unfortunately, this leads to a lot of fixup commits in the branch that muddle up the history. A changeset consists of one or more commits where each makes one logical change where the what was done and why it was done that way are detailed in the commit message.