Ruby からgit コマンドを実行して情報を取得
directory_path = '__my_git_repository_path__'
commit_id = '__commit_id__'
Dir.chdir(directory_path) do
git_show = `git show #{commit_id}`
p git_show.lines.select{ |line| line.start_with?('Date:') }.join.chomp
# => "Date: Sat Oct 4 02:00:04 2014 +0900"
end