Some fixes of the prompt of new branches (#26257)

Related to #26239

This PR makes some fixes:
- do not show the prompt for mirror repos and repos with pull request
units disabled
- use `commit_time` instead of `updated_unix`, as `commit_time` is the
real time when the branch was pushed

models/git/branch.go

@@ -395,9 +395,9 @@ func FindRecentlyPushedNewBranches(ctx context.Context, repoID, userID int64, ex
 		Where("pusher_id=? AND is_deleted=?", userID, false).
 		And("name <> ?", excludeBranchName).
 		And("repo_id = ?", repoID).
-		And("updated_unix >= ?", time.Now().Add(-time.Hour*6).Unix()).
+		And("commit_time >= ?", time.Now().Add(-time.Hour*6).Unix()).
 		NotIn("name", subQuery).
-		OrderBy("branch.updated_unix DESC").
+		OrderBy("branch.commit_time DESC").
 		Limit(2).
 		Find(&branches)
 	return branches, err