Ensure language is set on diffs from ActionLog and AcpThread (#46129)

Release Notes:

- N/A

crates/action_log/src/action_log.rs

@@ -79,7 +79,13 @@ impl ActionLog {
                 });
 
                 let text_snapshot = buffer.read(cx).text_snapshot();
-                let diff = cx.new(|cx| BufferDiff::new(&text_snapshot, cx));
+                let language = buffer.read(cx).language().cloned();
+                let language_registry = buffer.read(cx).language_registry();
+                let diff = cx.new(|cx| {
+                    let mut diff = BufferDiff::new(&text_snapshot, cx);
+                    diff.language_changed(language, language_registry, cx);
+                    diff
+                });
                 let (diff_update_tx, diff_update_rx) = mpsc::unbounded();
                 let diff_base;
                 let unreviewed_edits;