這個問題很容易解決，只要添加一個 .gitattributes
內容如下：
/////////////////////////////////////////////////////////////////////////
# Auto detect text files and perform LF normalization
* text=auto
# Custom for Visual Studio
*.cs diff=csharp
*.sln merge=union
*.csproj merge=union
*.vbproj merge=union
*.fsproj merge=union
*.dbproj merge=union
# Standard to msysgit
*.doc diff=astextplain
*.DOC diff=astextplain
*.docx diff=astextplain
*.DOCX diff=astextplain
*.dot diff=astextplain
*.DOT diff=astextplain
*.pdf diff=astextplain
*.PDF diff=astextplain
*.rtf diff=astextplain
*.RTF diff=astextplain
////////////////////////////////////////////////////////////////
git 完全支持 docx 文件的差異對比，因為 git 內部也是使用了 zlib，docx 本質上是 zip 文件,比較差異還是很容易的。
http://git-scm.com/book/en/v2/Customizing-Git-Git-Attributes