[css-grid] Specify how to handle two equal grid lines.

Author: tabatkins

css-grid/Overview.bs

@@ -2545,6 +2545,8 @@ Grid Placement Conflict Handling</h4>
 	If the <a>placement</a> for a <a>grid item</a> contains two lines,
 	and the <a>start</a> line is further end-ward than the <a>end</a> line,
 	swap the two lines.
+	If the <a>start</a> line is <em>equal</em> to the <a>end</a> line,
+	remove the <a>end</a> line.
 
 	If the <a>placement</a> contains two spans,
 	remove the one contributed by the <a>end</a> <a>grid-placement property</a>.