.git-completion.bash producing error on macOS Sierra 10.12.6
I ran into the exact same problem. After some digging, I finally figured out what the root problem is.

They made some major changes to the git-completion.bash script which requires a new feature in git v2.18, --list-cmds. The problem is that none of the package managers have updated to git v2.18 yet.

Most of the instructions out there say to download raw.githubusercontent.com/git/git/master/contrib/completion/git-completion.bash . But that is not necessarily the best option because sometimes you will be downloading a newer git-completion.bash than your version of git supports.

So the solution is to download the git-completion.bash version that matches your git version. Then source it again. In this case:

https://raw.githubusercontent.com/git/git/v2.17.1/contrib/completion/git-completion.bash

Notice that it is referencing v2.17.1 instead of master. Later, when you install git v2.18, then you can switch back to master, or v2.18 tag.