How can I set up autocompletion for Git commands?

I have Git (version 1.7.2.5) bash compeletion working on my Debian squeeze (6.0). Git was installed with aptitude and I am using standard debian's bash, which supports command line autocompletion.

Now, I just installed Git (1.5.6.5) on an other machine (Lenny/Debian 5.0) and the there is no autocompletion.

1. Why is Git autocomplete not working on the second machine? How do I diagnose this?

2. What is making completion work on my machine? I have looked for the file git-completion.bash but it doesn't seem to be on my machine. How does Git complete ever work?

3. How can I bring git complete to the other machine?

You need to source /etc/bash_completion.d/git to enable git auto-completion.

In my .bashrc it's done with:

for file in /etc/bash_completion.d/* ; do
    source "$file"
done

Put the following lines in your ~/.bashrc

if [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
fi

The script/program /etc/bash_completion already includes the scripts in /etc/bash_completion.d and also defines some functions needed by the included scripts.