Add the ability to propose changes to a set of buffers (#18170)

This PR introduces functionality for creating *branches* of buffers that
can be used to preview and edit change sets that haven't yet been
applied to the buffers themselves.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/remote_server/src/headless_project.rs

@@ -146,12 +146,15 @@ impl HeadlessProject {
         cx: &mut ModelContext<Self>,
     ) {
         match event {
-            BufferEvent::Operation(op) => cx
+            BufferEvent::Operation {
+                operation,
+                is_local: true,
+            } => cx
                 .background_executor()
                 .spawn(self.session.request(proto::UpdateBuffer {
                     project_id: SSH_PROJECT_ID,
                     buffer_id: buffer.read(cx).remote_id().to_proto(),
-                    operations: vec![serialize_operation(op)],
+                    operations: vec![serialize_operation(operation)],
                 }))
                 .detach(),
             _ => {}