Display head branch more comfortable on pull request view (#32000)

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)

models/issues/pull.go

@@ -268,6 +268,10 @@ func (pr *PullRequest) LoadAttributes(ctx context.Context) (err error) {
 	return nil
 }
 
+func (pr *PullRequest) IsAgitFlow() bool {
+	return pr.Flow == PullRequestFlowAGit
+}
+
 // LoadHeadRepo loads the head repository, pr.HeadRepo will remain nil if it does not exist
 // and thus ErrRepoNotExist will never be returned
 func (pr *PullRequest) LoadHeadRepo(ctx context.Context) (err error) {