[findbugs] Use explicit initialization

Findbugs doesn't like using implicitly initialized field in 
initializer.

Change-Id: Ic1ff9011813cc02950a71df587f31ed9f8415b49
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/util/BlockList.java

@@ -307,7 +307,7 @@ public class BlockList<T> extends AbstractList<T> {
 
 		private int blkIdx;
 
-		private T[] block = directory[dirIdx];
+		private T[] block = directory[0];
 
 		public boolean hasNext() {
 			return index < size;