git-review-rebase could find and warn about subsequent upstream commits fixing or reverting a commit in the rebased branch

[casasnovas]

When reviewing newly added commits in a patch-queue with git-review-rebase, we sometimes find that XenServer cherry-picked some upstream commits but overlooked to check if these commits were not introducing problems of their own.

It would nice if git-review-rebase could try and look to see if the newly added commits (or any commits for that matter), wasn't either reverted of fixed upstream.

[casasnovas]

Example of such cases can be found at:

• 8.3 import kernel-4.19.19 8.0.45.xs8.src.rpm xcp-ng-rpms/kernel#24 (comment)
• 8.3 import kernel 4.19.19 8.0.44.xs8.src.rpm xcp-ng-rpms/kernel#23

These were all manually found with git log --grep using either vanilla/master or the latest corresponding stable branch (e.g. cip/linux-4.9-cip.y).