Shut up findbugs/protect the shutdownHook in LocalDiskRepositoryTestcase

Singleton references should be protected from multiple threads. As far as we
know this cannot happen as JUnit is used today since we currently don't run
tests in parallel, but now this code will not prevent anyone.

Change-Id: I29109344d2e8025fa2a3ccaf7c2c16469544ce05
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/LocalDiskRepositoryTestCase.java

@@ -114,17 +114,18 @@ public abstract class LocalDiskRepositoryTestCase extends TestCase {
 	protected void setUp() throws Exception {
 		super.setUp();
 
-		if (shutdownHook == null) {
-			shutdownHook = new Thread() {
-				@Override
-				public void run() {
-					System.gc();
-					recursiveDelete("SHUTDOWN", trash, false, false);
-				}
-			};
-			Runtime.getRuntime().addShutdownHook(shutdownHook);
+		synchronized(this) {
+			if (shutdownHook == null) {
+				shutdownHook = new Thread() {
+					@Override
+					public void run() {
+						System.gc();
+						recursiveDelete("SHUTDOWN", trash, false, false);
+					}
+				};
+				Runtime.getRuntime().addShutdownHook(shutdownHook);
+			}
 		}
-
 		recursiveDelete(testName(), trash, true, false);
 
 		mockSystemReader = new MockSystemReader();