title: definition of gitk's "Mark Branch Sides" option
body: <p>In gitk, there is an option in the "Edit View" (F4) window called "Mark branch sides".  It seems to correspond to the --left-right option:</p>
  
  <pre><code>+set known_view_options {
  +    {perm    b    . {}               {mc "Remember this view"}}
  +    {args    t50= + {}               {mc "Commits to include (arguments to git log):"}}
  +    {all     b    * "--all"          {mc "Use all refs"}}
  +    {dorder  b    . {"--date-order" "-d"}      {mc "Strictly sort by date"}}
  +    {lright  b    . "--left-right"   {mc "Mark branch sides"}}
  </code></pre>
  
  <p>The only difference I can see is that the commits are marked by triangles instead of circles.  What is this option supposed to do?</p>