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)

routers/web/repo/pull.go

@@ -164,7 +164,19 @@ func setMergeTarget(ctx *context.Context, pull *issues_model.PullRequest) {
 		ctx.Data["HeadTarget"] = pull.MustHeadUserName(ctx) + "/" + pull.HeadRepo.Name + ":" + pull.HeadBranch
 	}
 	ctx.Data["BaseTarget"] = pull.BaseBranch
-	ctx.Data["HeadBranchLink"] = pull.GetHeadBranchLink(ctx)
+	headBranchLink := ""
+	if pull.Flow == issues_model.PullRequestFlowGithub {
+		b, err := git_model.GetBranch(ctx, ctx.Repo.Repository.ID, pull.HeadBranch)
+		switch {
+		case err == nil:
+			if !b.IsDeleted {
+				headBranchLink = pull.GetHeadBranchLink(ctx)
+			}
+		case !git_model.IsErrBranchNotExist(err):
+			log.Error("GetBranch: %v", err)
+		}
+	}
+	ctx.Data["HeadBranchLink"] = headBranchLink
 	ctx.Data["BaseBranchLink"] = pull.GetBaseBranchLink(ctx)
 }