Objective: Delete remote branches that have already been merged into master
# Make sure remotes are up to date (with stale remotes purged):
>>> git fetch -p
# Initial no-op run --- do the branches to delete look correct?
# Be careful to omit 'master' from the output.
>>> git branch --remote --merged origin/master | grep -v 'master' | cut -b 10- | xargs
# Do the bulk delete!!! (can take a long time...)
>>> git branch --remote --merged origin/master | grep -v 'master' | cut -b 10- | xargs git push --delete origin