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/channel/src/channel_buffer.rs

@@ -175,7 +175,10 @@ impl ChannelBuffer {
         cx: &mut ModelContext<Self>,
     ) {
         match event {
-            language::BufferEvent::Operation(operation) => {
+            language::BufferEvent::Operation {
+                operation,
+                is_local: true,
+            } => {
                 if *ZED_ALWAYS_ACTIVE {
                     if let language::Operation::UpdateSelections { selections, .. } = operation {
                         if selections.is_empty() {