copypatch: have dummy Writable object acts like a string (fixes #2280)
Since Mercurial 2.4, output (=template) parameter must be string . We can simply pass output='' for 2.4, but Mercurial 2.3 still requires dummy file-like object. As a workaround, this implements len(writable), so that it can bypass `len(template) > 0` of cmdutil.export().
I'll write another patch for default branch, which should take away dummy Writable hack.