Enhancement github releases generator

* Create GitHub Releases by itself
  * Added help and usage message
  * Decorate release body

tool/gen-github-release.rb

@@ -1,5 +1,11 @@
 #!/usr/bin/env ruby
 
+if ARGV.size < 2
+  puts "Usage: #{$0} <from version tag> <to version tag> [--no-dry-run]"
+  puts "     : if --no-dry-run is specified, it will create a release on GitHub"
+  exit 1
+end
+
 require "bundler/inline"
 
 gemfile do
@@ -19,6 +25,8 @@ end
 
 client = Octokit::Client.new
 
+note =  "## What's Changed\n\n"
+
 diff = client.compare("ruby/ruby", ARGV[0], ARGV[1])
 diff[:commits].each do |c|
   if c[:commit][:message] =~ /\[Backport #(\d*)\]/
@@ -32,8 +40,20 @@ diff[:commits].each do |c|
   else
     next
   end
-  puts "* [#{title}](#{url})"
+  note << "* [#{title}](#{url})\n"
 rescue OpenURI::HTTPError
   puts "Error: #{url}"
 end
 
+note << "\n"
+note << "Note: This list is automatically generated by tool/gen-github-release.rb. Because of this, some commits may be missing.\n\n"
+note << "## Full Changelog\n\n"
+note << "https://github.com/ruby/ruby/compare/#{ARGV[0]}...#{ARGV[1]}\n\n"
+
+if ARGV[2] == "--no-dry-run"
+  name = ARGV[1].gsub(/v/, "").gsub(/_/, ".")
+  client.create_release("ruby/ruby", ARGV[1], name: name, body: note)
+  puts "Created a release: https://github.com/ruby/ruby/releases/tag/#{ARGV[1]}"
+else
+  puts note
+end