In particular, only show robot comments from the latest patchset.

Bug: 924780
Change-Id: I12038ddd2d90a5cb561b248de3fd37908d5b927e
Reviewed-on: https://chromium-review.googlesource.com/c/1457282Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>