Don't reuse old syntax tree when resetting a buffer's language

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/language/src/buffer.rs

@@ -486,6 +486,7 @@ impl Buffer {
     }
 
     pub fn set_language(&mut self, language: Option<Arc<Language>>, cx: &mut ModelContext<Self>) {
+        *self.syntax_tree.lock() = None;
         self.language = language;
         self.reparse(cx);
     }