我有一个损坏的视频文件，其ffprobe（ffprobe -i movie.mkv）表示其持续时间约为2小时，但该文件仅包含前8分钟的视频数据（由VLC播放器检查）。
如何使用ffmpeg或ffprobe获得实际持续时间（8分钟）？
这是输出：
ffprobe version 0.7.13, Copyright (c) 2007-2011 the FFmpeg developers
built on Aug 1 2012 21:08:35 with clang 3.1 (tags/Apple/clang-318.0.58)
configuration: --prefix=/opt/local --enable-swscale --enable-avfilter --enable-libmp3lame --enable-libvorbis --enable-libtheora --enable-libdirac --enable-libschroedinger --enable-libopenjpeg --enable-libvpx --enable-libspeex --disable-libopencore-amrnb --disable-libopencore-amrwb --mandir=/opt/local/share/man --enable-shared --enable-pthreads --cc=/usr/bin/clang --arch=x86_64 --enable-yasm --enable-gpl --enable-postproc --enable-libx264 --enable-libxvid
libavutil 50. 43. 0 / 50. 43. 0
libavcodec 52.123. 0 / 52.123. 0
libavformat 52.111. 0 / 52.111. 0
libavdevice 52. 5. 0 / 52. 5. 0
libavfilter 1. 80. 0 / 1. 80. 0
libswscale 0. 14. 1 / 0. 14. 1
libpostproc 51. 2. 0 / 51. 2. 0
[matroska,webm @ 0x7f93dc01e600] Estimating duration from bitrate, this may be inaccurate
Input #0, matroska,webm, from 'Mission.mkv':
Duration: 02:12:56.21, start: 0.000000, bitrate: 448 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 1280x528 [PAR 1:1 DAR 80:33], 23.98 fps, 23.98 tbr, 1k tbn, 23.98 tbc (default)
Stream #0.1: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s (default)