IndexDiffSubmoduleTest: Fix negative use count

submoduleStandalone is created by createWorkRepository() which adds
the created repository to the set of repositories to be closed in
the test teardown. It is therefore not necessary to explicitly close
it.

Change-Id: Ib6f525b644fdeaaf1934df39cc2d3583a0d883dc
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/IndexDiffSubmoduleTest.java

@@ -90,7 +90,6 @@ public class IndexDiffSubmoduleTest extends RepositoryTestCase {
 				.setPath("modules/submodule")
 				.setURI(submoduleStandalone.getDirectory().toURI().toString())
 				.call();
-		submoduleStandalone.close();
 		submodule_trash = submodule_db.getWorkTree();
 		addRepoToClose(submodule_db);
 		writeTrashFile("fileInRoot", "root");