Fix DiffFormatter NPEs for DiffEntry without content change

DiffEntry.getOldId() returns null for a diff without an index line (e.g.
only mode changed, rename without content change).

Bug: 407743
Change-Id: I42eac87421f2a53c985af260a253338f578492bc

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/DiffFormatterTest.java

 /*
- * Copyright (C) 2010, Google Inc.
+ * Copyright (C) 2010, 2013 Google Inc.
  * and other copyright owners as documented in the project's IP log.
  *
  * This program and the accompanying materials are made available
 		assertEquals(0, hh.toEditList().size());
 	}
 
+	@Test
+	public void testCreateFileHeaderWithoutIndexLine() throws Exception {
+		DiffEntry m = DiffEntry.modify(PATH_A);
+		m.oldMode = FileMode.REGULAR_FILE;
+		m.newMode = FileMode.EXECUTABLE_FILE;
+
+		FileHeader fh = df.toFileHeader(m);
+		String expected = DIFF + "a/src/a b/src/a\n" + //
+				"old mode 100644\n" + //
+				"new mode 100755\n";
+		assertEquals(expected, fh.getScriptText());
+	}
+
+	@Test
+	public void testCreateFileHeaderForRenameWithoutContentChange() throws Exception {
+		DiffEntry a = DiffEntry.delete(PATH_A, ObjectId.zeroId());
+		DiffEntry b = DiffEntry.add(PATH_B, ObjectId.zeroId());
+		DiffEntry m = DiffEntry.pair(ChangeType.RENAME, a, b, 100);
+		m.oldId = null;
+		m.newId = null;
+
+		FileHeader fh = df.toFileHeader(m);
+		String expected = DIFF + "a/src/a b/src/b\n" + //
+				"similarity index 100%\n" + //
+				"rename from src/a\n" + //
+				"rename to src/b\n";
+		assertEquals(expected, fh.getScriptText());
+	}
+
 	@Test
 	public void testDiff() throws Exception {
 		write(new File(db.getDirectory().getParent(), "test.txt"), "test");

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

 			editList = new EditList();
 			type = PatchType.UNIFIED;
 
+		} else if (ent.getOldId() == null || ent.getNewId() == null) {
+			// Content not changed (e.g. only mode, pure rename)
+			editList = new EditList();
+			type = PatchType.UNIFIED;
+
 		} else {
 			assertHaveRepository();
 
 			o.write('\n');
 		}
 
-		if (!ent.getOldId().equals(ent.getNewId())) {
+		if (ent.getOldId() != null && !ent.getOldId().equals(ent.getNewId())) {
 			formatIndexLine(o, ent);
 		}
 	}