Add comment to original PR which tags all branch subscribers when a merge conflict auto-generated PR is created ("Merge master into <branch>")

[Julian88Tex]

Please describe the nature of this feature request

• I'm interested in functionality that solves a current problem where many auto-generated merge conflict PRs are not seen by appropriate parties.
• These PRs are created by a bot and always start with "Merge master into" followed by the branch name with the merge conflict. They occur when the newly updated master branch in an attempt to update child branches automatically after a plan completes with this step.

Describe the solution you'd like Based on a conversation with @davisagli, it sounds like the best solution may be adding a comment to the original PR.

Describe alternatives you've considered

• Alternative may be assigning the newly created PR to all assignees or 10 most recent subscribers (due to assignee limit) from original PR.
• Alternative may be adding subscribers to new PR based on subscriber from original PR and then adding a comment.

Additional context

• If there is a branch but no PR then perhaps using the branch creator (if possible) as the sole subscriber to the new PR is best (perhaps this is already happening).
• It would be helpful if the comment contained the following: -- The purpose of the auto-generated PR -- Next steps for resolving the conflict in Github -- Next steps for closing or resolving auto-generated PR

[davisagli]

@Julian88Tex I'm not sure I understand why tagging anyone is necessary. The subscribers will already get a notification, because they're subscribed.

[Julian88Tex]

@davisagli Oops meant to remove that. That was back from when I thought we were talking about commenting on the new PR and not the original PR. I've updated the Issue.