Create a .git_settings directory in (actually you can choose any):

Users/username/

copy following file from git sources:

git_source_files/contrib/completion/git-completion.bash

execute following command to edit bashrc file:

mate /etc/bashrc

and add following lines:

#git autocompletion
source ~/.git_settings/git-completion.bash

Now you can go to your repository and type:

git st<tab>