gistfile1.eclass

function changes {
  start="$1"
  end="$2"

  if [ -z "$2" ]; then
    start="$1"^
    end="$1"
  fi

  if [ -z "$1" ]; then
    start=HEAD^
    end=HEAD
  fi

  git diff --name-only $start..$end | sort | uniq
}

I came to the conclusion that your if-then logic is readable relative to any fancy shell parameter expansion I can come up with (which I gave up on). I think you have what you need to figure it out now.

For future reference I recommend the Advance Bash Scripting Guide - PDF / HTML versions at http://tldp.org/guides.html