By default, Git does not autocomplete commands and branch names.  If you have installed Git on Windows, autocomplete is available if you run the "Git Bash" program.  For OS X and Linux, you can install the auto-compelte tools and set up your environment to load them automatically.  See this page for instructions: https://git-scm.com/book/en/v1/Git-Basics-Tips-and-Tricks#Auto-Completion