In the latest segment of Xcode which was released recently have missing GIT Menu.

Press ⌘2 to open the new Source Control Navigator.
Right-click on the master icon.

To switch to a different branch

Right-click on a non-current branch
Choose Checkout…
Press ↩ or click on the Checkout button.

Expose the left side window (Navigator) in your Xcode by selecting this option (you will see this on the top right side of your Xcode window):