.gitconfig

[alias]
    cleanbranches = !/Users/jkakar/bin/gitcleanbranches.sh

gitcleanbranches.sh

#!/bin/bash

set -x
git checkout master
git pull origin master
for branch in `git branch -a --merged | grep -v HEAD | grep -v '*' | tr -d ' ' | cut -d / -f 3 -s | grep -v master`
do
    git branch -d $branch
    git push origin :refs/heads/$branch
done