This little script uses the git-branch-report command but creates a report showing the commands to delete local branches that have already been merged into master.

git-safe-delete-branch-report

#!/usr/bin/env ruby

require 'optparse'

options = {}

OptionParser.new do |opts|
  opts.banner = "Usage: git-safe-delete-branch-report [options]"

  opts.on("--porcelain", "Output machine-readable output") do |v|
    options[:porcelain] = v
  end
end.parse!

safe_branches = `git-branch-report --porcelain`.split("\n").grep(/master/).reject {|l|
  l =~ /^master/
}.map {|l| l.split(":")[0] }

if options[:porcelain]
  puts safe_branches.join("\n")
else
  if safe_branches.length > 0
    puts "The following branches have been merged into master and thus are safe to delete:"
    safe_branches.each do |branch|
      puts <<-EOM

  - #{branch}
    git branch -D #{branch}
  EOM
    end

    puts <<-EOM

  Delete all branches with the following command:
    git branch -D #{safe_branches.join(' ')}
  EOM
  else
    puts "There are no branches that are safe to delete"
  end
end