Exclude released PR on stable branch

[hsbt]

What was the end-user or developer problem that led to this PR?

I forgot to exclude released PRs from backport targets.

What is your fix for the problem, implemented in this PR?

Pick released PRs from commit log like:

❯ git log --oneline --grep "^Merge pull request #" v4.0.0..4.0
78bd794db8 (origin/4.0, 4.0) Merge pull request #9173 from ruby/release/4.0.1
165e7ebb88 Merge pull request #9169 from Shopify/ec-fix-clean
...
6ecf058112 Merge pull request #9152 from ruby/suppress-git-message
eea05a2510 Merge pull request #9150 from ruby/release/4.0.0

and scan #XXXX numbers. We already picked PR numbers from master branch for backporting targets. Now, we can select only #9178 in this time.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[Copilot]

Pull request overview

This PR adds logic to exclude already-released pull requests from backport targets when preparing patch releases on stable branches. The implementation scans the stable branch's commit log to identify PRs that have already been released and filters them out from the unreleased PR list.

• Adds released_pull_requests method to extract PR numbers from stable branch commits
• Updates unreleased_pr_ids to filter out released PRs during patch releases
• Changes block syntax for processing PR numbers from {...} to do...end

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The released_pull_requests method is called inside a loop for each PR number found (potentially hundreds of times), causing the git command to be executed repeatedly. This is very inefficient. Consider memoizing the result using ||= or moving the check outside the loop.

Suggested fix:

# In the released_pull_requests method definition (line 306-314):
def released_pull_requests
  @released_pull_requests ||= begin
    commits = `git log --oneline --grep "^Merge pull request #" #{@previous_release_tag}..#{@previous_stable_branch}`.split("\n")
    commits.filter_map do |commit|
      match = commit.match(/Merge pull request #(\d+) from /)
      match[1].to_i if match
    end
  end
end