clear-pr-merge-commit-message should preserve commit message if only a single commit is squashed #3140

We use "Squash & Merge" even for PRs with a single commit, because we require commits to be signed and GitHub does not sign commits when doing "Rebase & Merge". Thus, it would be great, if the commit message box would be pre-filled with the commit message of the first and only commit on the PR branch.