This PR do some minor improvements for head branch display on pull
request view UI.
- [x] Remove the link if the head branch has been deleted with a
tooltip, so that users will not result in a 404 page
- [x] Display a label if this pull request is an agit based one.
![图片](https://github.com/user-attachments/assets/872f26b6-f1cf-4427-9e41-e3a5b176dfa4)