ok, so I needed the git autocompletion script.
I got that from this url:
curl https://raw.githubusercontent.com/git/git/master/contrib/completion/git-completion.bash -o ~/.git-completion.bash
No need to worry about what directory you're in when you run this as your home directory(~) is used with the target.
Then I added to my ~/.bash_profile file the following 'execute if it exists' code:
if [ -f ~/.git-completion.bash ]; then
. ~/.git-completion.bash
fi
Update: I'm making these bits of code more concise to shrink down my .bashrc file, in this case I now use:
test -f ~/.git-completion.bash && . $_
Note: $_ means the last argument to the previous command. so . $_ means run it - "it" being .git-completion.bash in this case
This still works on both Ubuntu and OSX and on machines without the script .git-completion.bash script.
Now git Tab (actually it's git TabTab ) works like a charm!
(PS: If this doesn't work off the bat, you may need to run chmod -X ~/.git-completion.bash to give the script permission to run.)