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/assistant/src/context.rs

@@ -1006,9 +1006,12 @@ impl Context {
         cx: &mut ModelContext<Self>,
     ) {
         match event {
-            language::BufferEvent::Operation(operation) => cx.emit(ContextEvent::Operation(
-                ContextOperation::BufferOperation(operation.clone()),
-            )),
+            language::BufferEvent::Operation {
+                operation,
+                is_local: true,
+            } => cx.emit(ContextEvent::Operation(ContextOperation::BufferOperation(
+                operation.clone(),
+            ))),
             language::BufferEvent::Edited => {
                 self.count_remaining_tokens(cx);
                 self.reparse(cx);