It’s one step more indirect than that. If I want my prompt to tell me what branch is checked out, I can have it include the output of running `git branch`, for example. Unbeknownst to me, running `git branch` can cause git to run programs specified in the git repository’s config file. It’s not normally a problem of course, because I am using my own computer with all of my own git repositories. But it can be a problem if the computer is shared with others.
I think `git branch` here wasn't intended to be taken literally. If anything, you'd use a plumbing command to get the branch, not a porcelain command. I think they just meant that some commands that might be run might in turn run programs specified in the config file (either now or in the future).
