Easier mechanism for reviewing pull request changes (after initial review pass)

The current pull request feature is great for the initial review pass, but it is difficult to review subsequent changes to a pull request (eg. changes made in response to review feedback).

For longer pull requests, our developers typically make initial pass comments using the pull request, and then review individual commits for subsequent changes. This has a couple problems:

• If a submitter makes 2 (or more) corrective commits, the second commit may nullify portions of the first. So the reviewer wastes time reviewing (and commenting on) both.
• Comments on individual commits aren't displayed in the pull requests 'Activity' tab, making it difficult to ensure that all feedback has been addressed.

Ideas:

• Possibly some way to set the root commit used for the pull request diff (so reviewers can see only the aggregate result of the subsequent commits that haven't been reviewed yet)
• Less ideal (but perhaps simpler): Show commit comments in the pull request 'Activity' tab