こんな感じ
git branch --format='%(refname:lstrip=2)%00%(upstream:track)' | awk -F'\0' '$2=="[gone]"{print $1}' | xargs -r git branch -D
alias にしておくと便利
prune-local = !git branch --format='%(refname:lstrip=2)%00%(upstream:track)' | awk -F'\\0' '$2==\"[gone]\"{print $1}' | xargs -r git branch -D