Delete old preview branches if corresponding public preview branch is deleted

[epananth]

• This issue is blocking
• This issue is causing unreasonable pain

Automation request - When mirror detects that a public preview branch has been deleted, it should delete the corresponding internal branch.

FR convo

[wtgodbe]

This would be helpful for repos that have scheduled jobs w/ release/* as a branch trigger - since devs can't delete internal release branches, those pipelines are doing a bunch of pointless work on stale preview branches. e.g:

[mmitche]

Yep agreed this would be good. This would be:

• If a fast-forward merge and
• The public branch does not exist for a corresponding internal branch
• Delete the internal branch

We might want to initially add more checking..only delete if the head SHA of the branch is tagged.