Since 1.7.11 git subtree has been released as an official contrib. Here are commands that I had to type in into my Terminal in order to makes this script available:

$ cd /usr/local/share/git-core/contrib/subtree
$ make
$ sudo make prefix=/usr/local/Cellar/git/1.7.12 install

I think that theses commands may be more accurate (prefix is dependent to current version of git that is currently installed) but does the job. Hopes this help.