After launching Dreamweaver 2017.5 on macOS, when you click the Git icon in the Files panel, you see a pop-up message asking you to install Xcode command line.
By default, Git is not available in macOS. Git is available with Xcode command line tools. When you run the git command on a new machine, it executes usr/bin/git. If Xcode command Line tools is already installed, usr/bin/git works as git, else you see a popup asking you to install Xcode CommandLine Tools.
Install the Git client and then launch Dreamweaver. You can install the git client from here.