When you want to use git to backup your local documents on an usb stick, you can do the following steps.

1) Create Empty Git Repository

mkdir /Volumes/usb-stick/Documents.git
cd /Volumes/usb-stick/Documents.git
git --bare init

2) Create A First Backup Your Documents

cd ~/Documents
git init
git add ./
git commit
git remote add origin /Volumes/usb-stick/Documents.git
git push -u origin master

3) Backup New Files

cd ~/Documents
git add .
git commit
git push

4) You Can Also Clone Your Backup On Another Machine Or Folder

mkdir ~/Desktop/NewFolder
cd ~/Desktop/NewFolder
git clone /Volumes/usb-stick/Documents.git .