fetch_command: bypass export3() and checkout manually This method has several advantages: - export3() does not work very well under Windows, while client.list() and ra.get_file() do - File modes are retrieved from get_file() for free, instead of being read from the filesystem, which does not work under Windows, more generally the filesystem is bypassed completely. - It can be made much smarter by checkouting changed files only, like convert extension does.