Make sure checkout doesn't report conflicts on ignored paths

In a situation where a certain path was ignored but a working tree file
with this path existed jgit didn't allow to checkout a branch which
didn't ignore this path but contained different content. JGit considered
this to be a checkout conflict to prevent overwriting the file in the
working tree and raised an error. This commit fixes this by ensuring
that ignored dirty working tree files don't lead to a checkout conflict.

Bug: 450169
Change-Id: I90288d314ffac73c24a9c70a5181f8243bd4679a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -985,6 +985,38 @@ public class DirCacheCheckoutTest extends RepositoryTestCase {
 		}
 	}
 
+	@Test
+	public void testOverwriteUntrackedIgnoredFile() throws IOException,
+			GitAPIException {
+		String fname="file.txt";
+		Git git = Git.wrap(db);
+
+		// Add a file
+		writeTrashFile(fname, "a");
+		git.add().addFilepattern(fname).call();
+		git.commit().setMessage("create file").call();
+
+		// Create branch
+		git.branchCreate().setName("side").call();
+
+		// Modify file
+		writeTrashFile(fname, "b");
+		git.add().addFilepattern(fname).call();
+		git.commit().setMessage("modify file").call();
+
+		// Switch branches
+		git.checkout().setName("side").call();
+		git.rm().addFilepattern(fname).call();
+		writeTrashFile(".gitignore", fname);
+		git.add().addFilepattern(".gitignore").call();
+		git.commit().setMessage("delete and ignore file").call();
+
+		writeTrashFile(fname, "Something different");
+		git.checkout().setName("master").call();
+		assertWorkDir(mkmap(fname, "b"));
+		assertTrue(git.status().call().isClean());
+	}
+
 	@Test
 	public void testFileModeChangeWithNoContentChangeUpdate() throws Exception {
 		if (!FS.DETECTED.supportsExecute())

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

@@ -698,7 +698,7 @@ public class DirCacheCheckout {
 			// Nothing in Index
 			// At least one of Head, Index, Merge is not empty
 			// make sure not to overwrite untracked files
-			if (f != null) {
+			if (f != null && !f.isEntryIgnored()) {
 				// A submodule is not a file. We should ignore it
 				if (!FileMode.GITLINK.equals(mMode)) {
 					// a dirty worktree: the index is empty but we have a