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 >