What is it?
Use the gitignore file on your git repository to ignore all of the Joomla core files. The master branch will always keep the .gitignore file for the latest version of Joomla.
Just drop the gitignore file into a git repository that has Joomla on it. Rename it to .gitignore It will ignore all of the Joomla files and allow you to focus on the project that you are developing.
Looking for older versions of Joomla?
Check the Tags.