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/project/src/project.rs

@@ -2182,7 +2182,10 @@ impl Project {
 
         let buffer_id = buffer.read(cx).remote_id();
         match event {
-            BufferEvent::Operation(operation) => {
+            BufferEvent::Operation {
+                operation,
+                is_local: true,
+            } => {
                 let operation = language::proto::serialize_operation(operation);
 
                 if let Some(ssh) = &self.ssh_session {
@@ -2267,7 +2270,7 @@ impl Project {
                 .filter_map(|buffer| {
                     let buffer = buffer.upgrade()?;
                     buffer
-                        .update(&mut cx, |buffer, cx| buffer.git_diff_recalc(cx))
+                        .update(&mut cx, |buffer, cx| buffer.recalculate_diff(cx))
                         .ok()
                         .flatten()
                 })