The instructions for the 'advance testing setup' at the end of chapter 3 (3.7) of the book online suggest to checkout to the master branch before making changes.
Would not be convenient to add and commit these changes with git add -A and git commit -m? I am wondering whether there is some reason for these commands to be omitted at this stage, or they are supposed to be implicit.
No there where no good reasons for omitting them and either he forget about them or implies them, so use them as long as you can;)