NoteEditor Discard Changes: Do a git checkout on the file

This way it will go back to the exact original version. Our Note loading
and saving sometimes changes the yaml metadata order.

Also, this allows us to autosave the note file.

lib/screens/note_editor.dart

@@ -373,10 +373,9 @@ class NoteEditorState extends State<NoteEditor> {
     }
   }
 
-  void _discardChangesSelected(Note note) {
-    if (_noteModified(note)) {
-      note.data = originalNoteData;
-    }
+  void _discardChangesSelected(Note note) async {
+    var stateContainer = Provider.of<StateContainer>(context, listen: false);
+    stateContainer.discardChanges(note);
 
     Navigator.pop(context);
   }

lib/state_container.dart

@@ -413,6 +413,12 @@ class StateContainer with ChangeNotifier {
       _loadNotes();
     }
   }
+
+  Future<void> discardChanges(Note note) async {
+    var repo = await GitRepository.load(repoPath);
+    await repo.checkout(note.filePath);
+    return note.load();
+  }
 }
 
 Future<void> _copyDirectory(String source, String destination) async {