fatal: could not create work tree dir 'kivy'
You should do the command in a directory where you have write permission. So:
cd ~/
mkdir code
cd code
git clone https://github.com/kivy/kivy
For example.
Your current directory does not has the write/create permission to create kivy
directory, thats why occuring this problem.
Your current directory give 777 rights and try it.
sudo chmod 777 DIR_NAME
cd DIR_NAME
git clone https://github.com/mygitusername/kivy.git
In my case what happened was that the user I was using had no ownership over the directory. I simply had to change ownership of the directory to that user. For example if user is ubuntu:
chown ubuntu:ubuntu -R directory-in-question
cd directory-in-question/
git clone <git repo comes here >