RefTreeBatch: fix unclosed resource warning

Change-Id: I7d630d11d3ef2ff2b74ec7a7018760b7b4931ad2
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftree/RefTreeBatch.java

@@ -88,8 +88,9 @@ class RefTreeBatch extends BatchRefUpdate {
 			tree = RefTree.read(rw.getObjectReader(), c.getTree());
 		} else {
 			parentCommitId = ObjectId.zeroId();
-			parentTreeId = new ObjectInserter.Formatter()
-					.idFor(OBJ_TREE, new byte[] {});
+			try (ObjectInserter.Formatter fmt = new ObjectInserter.Formatter()) {
+				parentTreeId = fmt.idFor(OBJ_TREE, new byte[] {});
+			}
 			tree = RefTree.newEmptyTree();
 		}
 	}