I found this

Somewhere on the Internet. It was purportedly written by @spearce (aka Shawn O. Pearce), but I looked in his repos for several seconds and didn't see it.

It's GPL

At least, it is according to the comment docs.

It's wicked useful

git checkout r<tab> autocompletes to my remote of "rapid7"

Add this to your .bash_aliases

if [ -f /path/to/git-completion.bash ]; then
    . /path/to/git-completion.bash
fi

Zsh compatibility

Common error: "missing compdef"

autoload -U compinstall && compinstall

http://superuser.com/questions/320207/where-is-my-zsh-getting-its-git-completion-settings