Allow merge commits in stable branches

Author: k0kubun

bin/prohibit-merge-commits.rb

@@ -6,8 +6,9 @@
   out, _err, status = Open3.capture3("git", "rev-list", "--parents", "#{oldrev}..#{newrev}")
   if status.success?
     out.lines.each do |s|
-      if s.split.size >= 3
-        STDERR.puts "A merge commit is prohibited."
+      # Allow merge commits in stable branches to preserve history for stable releases
+      if s.split.size >= 3 && !File.basename(refname).match?(/\Aruby_\d+_\d+\z/)
+        STDERR.puts "A merge commit is prohibited for #{refname}."
         exit 1
       end
     end