in osx mountain lion, when you install xcode, the commandline version of git is not automatically installed.
to install git, follow these instructions:
1) make sure you have installed xcode from the app store
2) open xcode
3) goto xcode > preferences
4) goto downloads
5) tick ‘commandline tools’
xcode will download and configure the commandline tools for you and you will now have git available from the command line