Since Xcode hard coded its own version of git which is installed on /Applications/Xcode.app/Contents/Developer/usr/bin/git, I managed to use this work around trick:
- change into the Xcode directory: - cd /Applications/Xcode.app/Contents/Developer/usr/bin
 
- rename the Xcode's git like this: - sudo mv ./git ./git-xcode-usr-bin
 
- link my own git which is installed through homebrew: - sudo ln -s /usr/local/bin/git ./git
 
And I did the same thing with /usr/bin/git
This will acctually link /usr/local/Cellar/git/1.8.0/bin/git (because I'm use git 1.8.0 at the present)
Certainly this may cause some other problems, such as when I upgrade the homebrew's verion git in the future, it would not work for me :( and I have to make a new link then.
I do it like this because I want to solve my own problem here 13177203. And after search StackOverFlow for a long time, I finally got this solution.