Don't fetch Mirror when it's migrating (#19588)

- When a repository is still being migrated, don't try to fetch the
Mirror from the database. Instead skip it. This allows to visit
repositories that are still being migrated and were configured to be
mirrored.
- Resolves #19585
- Regression: #19295

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/task.go

@@ -181,6 +181,14 @@ func GetMigratingTask(repoID int64) (*Task, error) {
 	return &task, nil
 }
 
+// HasMigratingTask returns if migrating task exist for repo.
+func HasMigratingTask(repoID int64) (bool, error) {
+	return db.GetEngine(db.DefaultContext).Exist(&Task{
+		RepoID: repoID,
+		Type:   structs.TaskTypeMigrateRepo,
+	})
+}
+
 // GetMigratingTaskByID returns the migrating task by repo's id
 func GetMigratingTaskByID(id, doerID int64) (*Task, *migration.MigrateOptions, error) {
 	task := Task{