singleton NullChangeLogParser to reduce memory footprint

core/src/main/java/hudson/model/AbstractBuild.java

@@ -563,7 +563,7 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
                     // for historical reasons, null in the scm field means CVS, so we need to explicitly set this to something
                     // in case check out fails and leaves a broken changelog.xml behind.
                     // see http://www.nabble.com/CVSChangeLogSet.parse-yields-SAXParseExceptions-when-parsing-bad-*AccuRev*-changelog.xml-files-td22213663.html
-                    AbstractBuild.this.scm = new NullChangeLogParser();
+                    AbstractBuild.this.scm = NullChangeLogParser.INSTANCE;
 
                     try {
                         if (project.checkout(AbstractBuild.this,launcher,listener,new File(getRootDir(),"changelog.xml"))) {

core/src/main/java/hudson/scm/NullChangeLogParser.java

@@ -34,7 +34,14 @@ import java.io.IOException;
  * @author Kohsuke Kawaguchi
  */
 public class NullChangeLogParser extends ChangeLogParser {
+    
+    public static final NullChangeLogParser INSTANCE = new NullChangeLogParser();
+    
     public ChangeLogSet<? extends ChangeLogSet.Entry> parse(AbstractBuild build, File changelogFile) throws IOException, SAXException {
         return ChangeLogSet.createEmpty(build);
     }
+    
+    public Object readResolve() {
+        return INSTANCE;
+    }
 }

core/src/main/java/hudson/scm/NullSCM.java

@@ -55,7 +55,7 @@ public class NullSCM extends SCM {
     }
 
     public ChangeLogParser createChangeLogParser() {
-        return new NullChangeLogParser();
+        return NullChangeLogParser.INSTANCE;
     }
 
     @Extension