Fix conflict marker in project diff view (#26466)

Closes #ISSUE

Release Notes:

- N/A

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -2147,6 +2147,7 @@ impl MultiBuffer {
         }
 
         self.sync_diff_transforms(&mut snapshot, edits, DiffChangeKind::BufferEdited);
+        self.buffer_changed_since_sync.replace(true);
         cx.emit(Event::Edited {
             singleton_buffer_edited: false,
             edited_buffer: None,