[findBugs] Implement Serializable interface in PlotLane

This class is referenced by PlotCommit which is Serializable

Change-Id: If79b6a593dc53fbb7c4f0a69a9d0dca02a8cb63c

org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotLane.java

@@ -43,12 +43,16 @@
 
 package org.eclipse.jgit.revplot;
 
+import java.io.Serializable;
+
 /**
  * A line space within the graph.
  * <p>
  * Commits are strung onto a lane. For many UIs a lane represents a column.
  */
-public class PlotLane {
+public class PlotLane implements Serializable {
+	private static final long serialVersionUID = 1L;
+
 	int position;
 
 	/**