Reduce noise in PR diffs

[scholtzan]

The diff that gets automatically posted to PRs has been getting more noisy now that we de-activated that PRs need to be up-to-date with main. The diff tends to show changes that weren't made in the PR, but got pushed to main after the PR was last synced with main. To reduce the noise, we could do a pseudo merge for the diff and combine main branch with the PR branch to get the most recent diff. However, this would not work for merge conflicts.

┆Issue is synchronized with this Jira Task