Fix error occurring during checkout

Fix NullPointerException occurring when calling
CheckoutCommand with forced == true option when
the branch isn't changed and there is deleted
uncommitted file.

Change-Id: I99bf1fc25e6889f07092320d7bc2772ec5d341b5
Signed-off-by: Nail Samatov <sanail@yandex.ru>

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

@@ -124,6 +124,28 @@ public class CheckoutCommandTest extends RepositoryTestCase {
 				.setForced(true).call().getObjectId());
 	}
 
+	@Test
+	public void testCheckoutForced_deleteFileAndRestore() throws Exception {
+		File testFile = new File(db.getWorkTree(), "Test.txt");
+		assertTrue(testFile.exists());
+
+		assertEquals("test", git.getRepository().getBranch());
+		FileUtils.delete(testFile);
+		assertFalse(testFile.exists());
+		// Switch from "test" to "master".
+		assertEquals(initialCommit.getId(), git.checkout().setName("master")
+				.setForced(true).call().getObjectId());
+		assertTrue(testFile.exists());
+
+		assertEquals("master", git.getRepository().getBranch());
+		FileUtils.delete(testFile);
+		assertFalse(testFile.exists());
+		// Stay in current branch.
+		assertEquals(initialCommit.getId(), git.checkout().setName("master")
+				.setForced(true).call().getObjectId());
+		assertTrue(testFile.exists());
+	}
+
 	@Test
 	public void testCreateBranchOnCheckout() throws Exception {
 		git.checkout().setCreateBranch(true).setName("test2").call();

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -1217,7 +1217,7 @@ public class DirCacheCheckout {
 		if (e != null && !FileMode.TREE.equals(e.getFileMode()))
 			builder.add(e);
 		if (force) {
-			if (f.isModified(e, true, walk.getObjectReader())) {
+			if (f == null || f.isModified(e, true, walk.getObjectReader())) {
 				kept.add(path);
 				checkoutEntry(repo, e, walk.getObjectReader(), false,
 						new CheckoutMetadata(walk.getEolStreamType(CHECKOUT_OP),