分两步走~
step 1.
create new .sh file as Git's diff warpper.
#!/bin/sh
/cygdrive/c/Program\ Files\ \(x86\)/Beyond\ Compare\ 3/BCompare.exe "d:/programs/cygwin$2" "$5"
** please notice here, you need specially make a full path of $2, since BC3 will not find your tmp file under Cygwin path. **
** 注意这里的两个路径，第二个是本地文件，因为是相对路径，BC3可以找到，但第一个是绝对路径指向/tmp/xxx，所以我们要特别指定在windows下的真实路径，而不是cygwin的路径，所以我加上了d:/programs/cygwin。
step 2.
Add this line to .bashrc and run it to configure your Git gloabl variable.
git config --global diff.external /home/z33/git-diff-wrapper.sh
Now you can try git diff whatever your file.
