buffer_diff: Reduce number of entity clones in base_text_buffer (#49573)

Release Notes:

- Improved split performance by reducing number of entity clones for
very large pre-loaded multibuffers.

crates/buffer_diff/src/buffer_diff.rs

@@ -1978,8 +1978,8 @@ impl BufferDiff {
         cx.emit(BufferDiffEvent::DiffChanged(change));
     }
 
-    pub fn base_text_buffer(&self) -> Entity<language::Buffer> {
-        self.inner.base_text.clone()
+    pub fn base_text_buffer(&self) -> &Entity<language::Buffer> {
+        &self.inner.base_text
     }
 }