`update_generation_config.sh` does not consider existing closed PRs when checking for branch existence

[diegomarquezp]

For example: https://github.com/googleapis/java-pubsub/blob/ce7cb09ac5e86953815e2ead9ae9202e9b034ba6/.github/scripts/update_generation_config.sh#L109 is used to check whether we have an open PR with the target branch name, but unluckily java-pubsub had a closed PR with that branch name, so the whole action failed due to a git push that conflicted with the remote's contents.

An easy workaround: search for closed PRs (in this case it was this one) and run the action again.

[zhumin8]

Thanks for pointing out the exact logic accounting for this behavior. I feel we opened similar issues before.
My understanding this was the intended behavior for the design, but the error should be more specific -- workflow should fail if target branch exists but not associated with a PR. I expect the current error be: fatal: A branch named 'generate-libraries-<your_base_branch>' already exists.
For clarity, I suppose we can check explicitly and leave a bit more context in the log. I wonder if that would help?

if [ -z "${pr_num}" ]; then
    # Check if the branch exists on the remote origin
    if git ls-remote --exit-code --heads origin "${current_branch}" >/dev/null 2>&1; then
        echo "Error: Branch '${current_branch}' exists on remote, but no open PR was found."
        echo "Please delete the remote branch or reopen the PR."
        exit 1
    fi

    # Safe to create the branch now
    git checkout -b "${current_branch}"
    # Push the current branch to remote so that we can compare the commits later.
    git push -u origin "${current_branch}"
else

For the time-being, the above workaround is fine.