Replace DiffCommand with IndexDiff

DiffCommand suffered from not taking smudge/clean filters into account

plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java

@@ -19,16 +19,14 @@
 import java.io.IOException;
 import java.nio.file.Path;
 import java.nio.file.Paths;
-import java.util.List;
+import java.util.Collection;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
-import org.eclipse.jgit.api.Git;
-import org.eclipse.jgit.api.errors.GitAPIException;
-import org.eclipse.jgit.diff.DiffEntry;
+import org.eclipse.jgit.lib.IndexDiff;
 import org.eclipse.jgit.lib.ObjectId;
-import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Repository;
-import org.eclipse.jgit.treewalk.CanonicalTreeParser;
+import org.eclipse.jgit.treewalk.FileTreeIterator;
 
 import com.diffplug.spotless.extra.GitRatchet;
 
@@ -58,26 +56,23 @@ static GitRatchetMaven instance() {
 		return instance;
 	}
 
-	Iterable<String> getDirtyFiles(File baseDir, String ratchetFrom)
-			throws IOException, GitAPIException {
+	Iterable<String> getDirtyFiles(File baseDir, String ratchetFrom) throws IOException {
 		Repository repository = repositoryFor(baseDir);
 		ObjectId sha = rootTreeShaOf(baseDir, ratchetFrom);
 
-		ObjectReader oldReader = repository.newObjectReader();
-		CanonicalTreeParser oldTree = new CanonicalTreeParser();
-		oldTree.reset(oldReader, sha);
-
-		Git git = new Git(repository);
-		List<DiffEntry> diffs = git.diff()
-				.setShowNameAndStatusOnly(true)
-				.setOldTree(oldTree)
-				.call();
+		IndexDiff indexDiff = new IndexDiff(repository, sha, new FileTreeIterator(repository));
+		indexDiff.diff();
 
 		String workTreePath = repository.getWorkTree().getPath();
 		Path baseDirPath = Paths.get(baseDir.getPath());
 
-		return diffs.stream()
-				.map(DiffEntry::getNewPath)
+		return Stream.of(
+				indexDiff.getAdded(),
+				indexDiff.getChanged(),
+				indexDiff.getConflicting(),
+				indexDiff.getModified(),
+				indexDiff.getUntracked())
+				.flatMap(Collection::parallelStream)
 				.map(path -> Paths.get(workTreePath, path))
 				.map(path -> baseDirPath.relativize(path).toString())
 				.collect(Collectors.toList());