CommitOnlyTest: Open RevWalk in try-with-resource

Change-Id: Ia3db8696f66f8d294e1d443fb54e716d26d517b1
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/CommitOnlyTest.java

@@ -1294,10 +1294,12 @@ public class CommitOnlyTest extends RepositoryTestCase {
 		try {
 			final Repository repo = git.getRepository();
 			final ObjectId headId = repo.resolve(Constants.HEAD + "^{commit}");
+			try (RevWalk rw = new RevWalk(repo)) {
 				final TreeWalk tw = TreeWalk.forPath(repo, path,
-					new RevWalk(repo).parseTree(headId));
+						rw.parseTree(headId));
 				return new String(tw.getObjectReader().open(tw.getObjectId(0))
 						.getBytes());
+			}
 		} catch (Exception e) {
 			return "";
 		}