function git-diff-branches() {
  branch1="$1"
  branch2="$2"

  if [ -z "$1" -a -z "$2" -a "$(git branch | head -1 | awk '{print $2}')" = "master" ]; then
    branch1="master"
    branch2="origin/master"
  else
    if [ -z "$1" -o -z "$2" ]; then
      echo "git-diff-branches <branch1> <branch2>" >>/dev/stderr
      return
    fi
  fi

  echo "Commits on $branch1 that aren't on $branch2\n----"
  git log $(git merge-base HEAD $branch2)..$branch1
  echo

  echo "Commits on $branch2 that aren't on $branch1\n----"
  git log $(git merge-base HEAD $branch1)..$branch2
}