Git autocompletion is not working with homebrew
For Yosemite, after I upgraded to git 2.1.3 with homebrew, this worked in my .profile file (.bash_profile or similar would work, too):

# git tab completion (homebrew)
if [ -f `brew --prefix`/etc/bash_completion.d/git-completion.bash ]; then
    . `brew --prefix`/etc/bash_completion.d/git-completion.bash
fi


I used to have git-prompt.sh there, but changing it to git-completion.bash worked for me.  ymmv.