Review backport PR diffs

[mattclay]

PRs for stable branches should be reviewed by the bot to determine if the diff is an exact match for the original PR in devel. If there is a match, apply a label, perhaps something like exact_backport to the PR. If commits are added/changed, the bot should re-evaluate the PR and remove the label if it no longer applies.

The comparison should ignore a missing changelog fragment in the devel diff. This will handle the situation where the changelog fragment was only added in the backport PR.

This will reduce the burden on release managers reviewing backport PRs, as most PRs cherry pick cleanly, making the comparison easy for the bot (assuming -x was used to identify the original).

[abadger]

Other things to check on backports:

• Presence of a changelog fragment
• That a PR to devel for the same issue has been merged (or some subset of this... That a PR to devel for this issue is not open. That a PR to devel referenced in the ticket that looks like it's related is not merged. That a PR to devel by the author for this issue has not been merged).