on Mac OS X you do not have tab-completion in git. You have to activate it. Recently my colleauge Enrico found a description how to do so:
This is cool but it will not work, when you have installed git via homebrew … what I strongly recommend. Here is what you have to add to your .bash_profile in your homedirectory:
Make sure that the two files are executable (change it with chmod a+x [file]).