git delete all branches that are merged into current branch
git branch --merged | grep -v "*" | grep -v master | grep -v develop | xargs -n 1 git branch -d
References
https://gist.github.com/mizanRahman/3a7887a2eaac789677579959b1dc9f5c
http://stackoverflow.com/questions/6127328/how-can-i-delete-all-git-branches-which-have-been-merged