There are files that should not be included in Git repositories (like IDE settings, OS generated files, temporary files etc.). Instead of adding them to the .gitignore file for each repository over again you can use a global .gitignore file:
Create a .gitignore_global file in your home directory (subl ~/.gitignore_global) with the following content:
# OS files
.DS_Store
.DS_Store?
.Spotlight-V100
.Trashes
Icon?