Support stable branch like 3.7/4.0

Author: hsbt

tool/release.rb

@@ -323,7 +323,13 @@ def scan_unreleased_pull_requests(ids)
   end
 
   def unreleased_pr_ids
-    commits = `git log --format=%h #{@previous_release_tag}..HEAD`.split("\n")
+    commits = if @level == :minor_or_major
+      `git log --format=%h #{@previous_release_tag}..HEAD`.split("\n")
+    else
+      `git log --format=%B #{@previous_release_tag}..HEAD`.split("\n").filter_map do |line|
+        line[/\(cherry picked from commit ([0-9a-f]+)\)/, 1]&.slice(0, 12)
+      end
+    end
 
     # GitHub search API has a rate limit of 30 requests per minute for authenticated users
     rate_limit = 28