Following on RELENG-4568 the PR update action (or bot) should appropriately update the relevant Gerrit change with new squash commits of the PR.

This is a follow-up to the discussion we have had at https://lists.opendaylight.org/g/TSC/topic/git_workflows/9292403...

What we need covered is reflecting GH PR requests to Gerrit, so they show up as contributions. As noted in the discussion, we are assuming ‘Allow edits by maintainers’ being set and with that we need a bot, which will:

• watch for PRs being created
• check if a Change-ID line is present in the PR message
• if not, assign a Change-ID by editing the PR (and leaving a message to that effect)
• create a Gerrit patch with the Change-ID, squashing all PR changes into a single commit
• monitor the PR for updates and update the Gerrit change when they happen (leaving a message on the PR)
• close the PR when the Gerrit patch is merged or abandoned, leaving the appropriate message pointing to the merged patch

The Gerrit → PR changes are explicitly out of scope as the commit squashing done in PR → Gerrit is not reversible by nature. We may revisit this point at a later date.