[BREAKING] remove diff display from 'task undo'

TaskChampion does not make the necessary information available to
accomplish this, but see #2928.

src/TDB2.cpp

@@ -780,8 +780,7 @@ void TDB2::revert ()
   std::string prior;
   revert_undo (u, uuid, when, current, prior);
 
-  // Display diff and confirm.
-  show_diff (current, prior, when);
+  // Confirm.
   if (! Context::getContext ().config.getBoolean ("confirmation") ||
       confirm ("The undo command is not reversible.  Are you sure you want to revert to the previous state?"))
   {

test/undo.t

@@ -74,13 +74,15 @@ class TestUndoStyle(TestCase):
         self.t("add one project:foo priority:H")
         self.t("1 modify +tag project:bar priority:")
 
+    @unittest.expectedFailure  # undo diffs are not supported
     def test_undo_side_style(self):
         """Test that 'rc.undo.style:side' generates the right output"""
         self.t.config("undo.style", "side")
         code, out, err = self.t("undo", input="n\n")
         self.assertNotRegex(out, "-tags:\s*\n\+tags:\s+tag")
         self.assertRegex(out, "tags\s+tag\s*")
 
+    @unittest.expectedFailure  # undo diffs are not supported
     def test_undo_diff_style(self):
         """Test that 'rc.undo.style:diff' generates the right output"""
         self.t.config("undo.style", "diff")