Set context classloader to null in WorkQueue

If threads created by the executor have a reference to the context
classloader this may cause a leak.

See: https://wiki.apache.org/tomcat/MemoryLeakProtection#cclThreadSpawnedByWebApp
Bug: 517107
Change-Id: If79c1ebd3f9cf733fd88a5fce38fd46ff0d08d10
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/internal/WorkQueue.java

@@ -71,6 +71,7 @@ public class WorkQueue {
 					public Thread newThread(Runnable taskBody) {
 						Thread thr = baseFactory.newThread(taskBody);
 						thr.setName("JGit-WorkQueue"); //$NON-NLS-1$
+						thr.setContextClassLoader(null);
 						thr.setDaemon(true);
 						return thr;
 					}