export: port _exportsingle() to formatter Pass 'fm' instead of 'write', and use fm.plain(), fm.write(), etc. instead. The callers will be updated later.

export: serialize revisions to be exported per destination file Prepares for porting to the formatter API, where we can't simply append to existing files because JSON can't be streamed for example. The modemap hack is removed since cmdutil.export() was the only user. I also made the destination filename printed only once.