dfs: actually allow current DfsBlock to GC

Holding the current DfsBlock in a local variable 'b' may prevent the
Java GC from reclaiming it while loading the next block.  Remove the
local variable and rely only on the field.

Change-Id: Ibfc8394cac717b485fdc94d5c8479c3f8ca78ee4

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsReader.java

@@ -757,8 +757,7 @@ public class DfsReader extends ObjectReader implements ObjectReuseAsIs {
 	}
 
 	void pin(DfsPackFile pack, long position) throws IOException {
-		DfsBlock b = block;
-		if (b == null || !b.contains(pack.key, position)) {
+		if (block == null || !block.contains(pack.key, position)) {
 			// If memory is low, we may need what is in our window field to
 			// be cleaned up by the GC during the get for the next window.
 			// So we always clear it, even though we are just going to set