Wrap the Files.list returned Stream in a try-with-resources block

Adds a new FileUtils.hasFiles(Path) helper method to correctly handle
the Files.list returned Stream.

These errors were found by compiling the code using JDK11's
javac compiler.

Change-Id: Ie8017fa54eb56afc2e939a2988d8b2c5032cd00f
Signed-off-by: Terry Parker <tparker@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java

 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
-import java.nio.file.Files;
 import java.text.MessageFormat;
 import java.text.ParseException;
 import java.util.ArrayList;
 			throws IOException {
 		File reftableDir = new File(getDirectory(), Constants.REFTABLE);
 		File headFile = new File(getDirectory(), Constants.HEAD);
-		if (reftableDir.exists()
-				&& Files.list(reftableDir.toPath()).findAny().isPresent()) {
+		if (reftableDir.exists() && FileUtils.hasFiles(reftableDir.toPath())) {
 			throw new IOException(JGitText.get().reftableDirExists);
 		}
 

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/WorkingTreeIterator.java

 import java.nio.CharBuffer;
 import java.nio.charset.CharacterCodingException;
 import java.nio.charset.CharsetEncoder;
-import java.nio.file.Files;
 import java.nio.file.Path;
 import java.text.MessageFormat;
 import java.time.Instant;
 import org.eclipse.jgit.treewalk.TreeWalk.OperationType;
 import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FS.ExecutionResult;
+import org.eclipse.jgit.util.FileUtils;
 import org.eclipse.jgit.util.Holder;
 import org.eclipse.jgit.util.IO;
 import org.eclipse.jgit.util.Paths;
 						idOffset) == 0) {
 					Path p = repository.getWorkTree().toPath()
 							.resolve(entry.getPathString());
-					return Files.list(p).findAny().isPresent();
+					return FileUtils.hasFiles(p);
 				}
 				return false;
 			} else if (mode == FileMode.SYMLINK.getBits())

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

 import java.util.Locale;
 import java.util.Random;
 import java.util.regex.Pattern;
+import java.util.stream.Stream;
 
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.Constants;
 		return Files.isRegularFile(file.toPath(), LinkOption.NOFOLLOW_LINKS);
 	}
 
+	/**
+	 * Whether the path is a directory with files in it.
+	 *
+	 * @param dir
+	 *            directory path
+	 * @return {@code true} if the given directory path contains files
+	 * @throws IOException
+	 *             on any I/O errors accessing the path
+	 *
+	 * @since 5.11
+	 */
+	public static boolean hasFiles(Path dir) throws IOException {
+		try (Stream<Path> stream = Files.list(dir)) {
+			return stream.findAny().isPresent();
+		}
+	}
+
 	/**
 	 * Whether the given file can be executed.
 	 *