I just installed Mac OS X Mountain Lion, as an update via the app store. Worked pretty good. After less than 30 min it was installed. Everything worked. I just missed the command line tool git.
To fix that issue I had to install the newest XCode version and XCode command line tools. The XCode command line tools you can install inside from XCode. Just go to preferences and Downloads and then you will see it.
That worked for me.