WIP - Serialize buffer in terms of its state, not its base text + ops

The main reason for this is that we need to include information about
a buffer's UndoMap into its protobuf representation. But it's a bit
complex to correctly incorporate this information into the current
protobuf representation.

If we want to continue reusing `Buffer::apply_remote_edit` for
incorporating the historical operations, we need to either make
that method capable of incorporating already-undone edits, or
serialize the UndoMap into undo *operations*, so that we can apply
these undo operations after the fact when deserializing. But this is
not trivial, because an UndoOperation requires information about
the full offset ranges that were undone.

crates/language/src/tests.rs

@@ -782,17 +782,30 @@ async fn test_empty_diagnostic_ranges(mut cx: gpui::TestAppContext) {
 
 #[gpui::test]
 fn test_serialization(cx: &mut gpui::MutableAppContext) {
+    let mut now = Instant::now();
+
     let buffer1 = cx.add_model(|cx| {
         let mut buffer = Buffer::new(0, "abc", cx);
-        buffer.edit([3..3], "DE", cx);
+        buffer.edit([3..3], "D", cx);
+
+        now += Duration::from_secs(1);
+        buffer.start_transaction_at(now);
+        buffer.edit([4..4], "E", cx);
+        buffer.end_transaction_at(now, cx);
+        assert_eq!(buffer.text(), "abcDE");
+
         buffer.undo(cx);
+        assert_eq!(buffer.text(), "abcD");
+
+        buffer.edit([4..4], "F", cx);
+        assert_eq!(buffer.text(), "abcDF");
         buffer
     });
-    assert_eq!(buffer1.read(cx).text(), "abc");
+    assert_eq!(buffer1.read(cx).text(), "abcDF");
 
     let message = buffer1.read(cx).to_proto();
     let buffer2 = cx.add_model(|cx| Buffer::from_proto(1, message, None, cx).unwrap());
-    assert_eq!(buffer2.read(cx).text(), "abc");
+    assert_eq!(buffer2.read(cx).text(), "abcDF");
 }
 
 fn chunks_with_diagnostics<T: ToOffset + ToPoint>(