Here's my take on the one-liner that I use via a `git tidy` alias[1]. A few points:
* It ensures the default branch is not deleted (main, master)
* It does not touch the current branch
* It does not touch the branch in a different worktree[2]
* It also works with non-merge repos by deleting the local branches that are gone on the remote
git branch --merged "$(git config init.defaultBranch)" \
| grep -Fv "$(git config init.defaultBranch)" \
| grep -vF '*' \
| grep -vF '+' \
| xargs git branch -d \
&& git fetch \
&& git remote prune origin \
&& git branch -v \
| grep -F '[gone]' \
| grep -vF '*' \
| grep -vF '+' \
| awk '{print $1}' \
| xargs git branch -D