Rake task for generating a change log based on git tags

changelog.rake

desc "Outputs a change log based on git tags"
task :changelog do
  tags = Dir['.git/refs/tags/*'].each.with_object({}) do |path, hsh|
    hsh[File.basename(path)] = File.read(path).chomp
  end

  tag_outputs = []
  tags.reduce(nil) do |(_, commit1), (name, commit2)|
    tag_date = `git log -1 --format="%ci" #{commit2}`.chomp
    lines = [ "## #{name} - #{tag_date}\n" ]
    if commit1
      lines << `git log #{commit1}...#{commit2} --pretty=" * (%h) %s [%an]"`
    end

    tag_outputs << lines.join("\n")
    [name, commit2]
  end
  puts tag_outputs.reverse.join("\n")
end