Make this executable and it will list for you the remote branches that haven't been merged into the "develop" branch. You can, of course, grep for a different branch name (master?) or change the || to && to get remote branches that ARE merged in.

unmerged-branches

#! /bin/sh

if [[ -z $1 ]]; then
  echo "You must provide a branch name against which to test merging."
  exit 1
fi

gather_branches() {
  for remote in `git for-each-ref --format='%(refname)' 'refs/remotes/'`
  do
    git branch --contains $remote | grep -q $1

    if [[ $? ]]; then
      branch_name=`echo $remote | sed -e's/refs\/remotes\///'`
    fi

    commit_time_output=`git log --format=format:'%ad,%cn,%at' $branch_name | head -n1`

    echo $branch_name$','$commit_time_output
  done
}

gather_branches $1 | sort -t',' -k4

Updated this to accept a command-line argument for which branch to compare against and then do some sorting and prettifying of output. Usage like this: unmerged-branches develop > look_into_these.csv.