Allow checkout paths without specifying branch name

JGit CLI should allow to do this: checkout -- <path>

Currently, even if "a" is a valid path in the git repo, jgit CLI can't
checkout it:
$jgit checkout -- a
error: pathspec 'a' did not match any file(s) known to git.

The fix also fixes at same time "unnamed" zombie "[VAL ...]" argument
shown on the command line.

Before fix:
$jgit -h
jgit checkout name [VAL ...] [-- path ... ...] [--force (-f)] [--help
(-h)] [--orphan] [-b]

After fix:
$jgit -h
jgit checkout [name] [-- path ... ...] [--force (-f)] [--help (-h)]
[--orphan] [-b]

Bug: 475765
Change-Id: I2b0e77959a72e4aac68452dc3846adaa745b0831
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit.pgm.test/src/org/eclipse/jgit/lib/CLIRepositoryTestCase.java

@@ -46,6 +46,7 @@ import static org.junit.Assert.assertEquals;
 
 import java.io.File;
 import java.io.IOException;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -76,6 +77,18 @@ public class CLIRepositoryTestCase extends LocalDiskRepositoryTestCase {
 		return result.toArray(new String[0]);
 	}
 
+	/**
+	 * @param link
+	 *            the path of the symbolic link to create
+	 * @param target
+	 *            the target of the symbolic link
+	 * @return the path to the symbolic link
+	 * @throws Exception
+	 */
+	protected Path writeLink(String link, String target) throws Exception {
+		return JGitTestUtil.writeLink(db, link, target);
+	}
+
 	protected File writeTrashFile(final String name, final String data)
 			throws IOException {
 		return JGitTestUtil.writeTrashFile(db, name, data);

org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/CheckoutTest.java

@@ -44,9 +44,14 @@ package org.eclipse.jgit.pgm;
 
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
 
 import java.io.File;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
 import java.util.List;
 
 import org.eclipse.jgit.api.Git;
@@ -59,7 +64,9 @@ import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.treewalk.FileTreeIterator;
 import org.eclipse.jgit.treewalk.FileTreeIterator.FileEntry;
 import org.eclipse.jgit.treewalk.TreeWalk;
+import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FileUtils;
+import org.junit.Assume;
 import org.junit.Test;
 
 public class CheckoutTest extends CLIRepositoryTestCase {
@@ -578,4 +585,34 @@ public class CheckoutTest extends CLIRepositoryTestCase {
 				execute("git branch"));
 		assertEquals("Hello world b", read(b));
 	}
+
+	@Test
+	public void testCheckouSingleFile() throws Exception {
+		try (Git git = new Git(db)) {
+			File a = writeTrashFile("a", "file a");
+			git.add().addFilepattern(".").call();
+			git.commit().setMessage("commit file a").call();
+			writeTrashFile("a", "b");
+			assertEquals("b", read(a));
+			assertEquals("[]", Arrays.toString(execute("git checkout -- a")));
+			assertEquals("file a", read(a));
+		}
+	}
+
+	@Test
+	public void testCheckoutLink() throws Exception {
+		Assume.assumeTrue(FS.DETECTED.supportsSymlinks());
+		try (Git git = new Git(db)) {
+			Path path = writeLink("a", "link_a");
+			assertTrue(Files.isSymbolicLink(path));
+			git.add().addFilepattern(".").call();
+			git.commit().setMessage("commit link a").call();
+			deleteTrashFile("a");
+			writeTrashFile("a", "Hello world a");
+			assertFalse(Files.isSymbolicLink(path));
+			assertEquals("[]", Arrays.toString(execute("git checkout -- a")));
+			assertEquals("link_a", FileUtils.readSymLink(path.toFile()));
+			assertTrue(Files.isSymbolicLink(path));
+		}
+	}
 }

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Checkout.java

@@ -60,7 +60,7 @@ import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.pgm.internal.CLIText;
 import org.kohsuke.args4j.Argument;
 import org.kohsuke.args4j.Option;
-import org.kohsuke.args4j.spi.StopOptionHandler;
+import org.kohsuke.args4j.spi.RestOfArgumentsHandler;
 
 @Command(common = true, usage = "usage_checkout")
 class Checkout extends TextBuiltin {
@@ -74,11 +74,10 @@ class Checkout extends TextBuiltin {
 	@Option(name = "--orphan", usage = "usage_orphan")
 	private boolean orphan = false;
 
-	@Argument(required = true, index = 0, metaVar = "metaVar_name", usage = "usage_checkout")
+	@Argument(required = false, index = 0, metaVar = "metaVar_name", usage = "usage_checkout")
 	private String name;
 
-	@Argument(index = 1)
+	@Option(name = "--", metaVar = "metaVar_paths", multiValued = true, handler = RestOfArgumentsHandler.class)
-	@Option(name = "--", metaVar = "metaVar_paths", multiValued = true, handler = StopOptionHandler.class)
 	private List<String> paths = new ArrayList<String>();
 
 	@Override