Don't use '-shortest' option for merging formats (closes #4220, closes #4580)

With avconv and older versions of ffmpeg the video is partially copied.
The duration difference between the audio and the video seem to be really small, so it's probably not noticeable.

youtube_dl/postprocessor/ffmpeg.py

@@ -520,7 +520,7 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 class FFmpegMergerPP(FFmpegPostProcessor):
     def run(self, info):
         filename = info['filepath']
-        args = ['-c', 'copy', '-map', '0:v:0', '-map', '1:a:0', '-shortest']
+        args = ['-c', 'copy', '-map', '0:v:0', '-map', '1:a:0']
         self._downloader.to_screen('[ffmpeg] Merging formats into "%s"' % filename)
         self.run_ffmpeg_multiple_files(info['__files_to_merge'], filename, args)
         return True, info