fix repo migrate bug

modules/repository/repo.go

@@ -139,6 +139,9 @@ func MigrateRepositoryGitData(doer, u *models.User, repo *models.Repository, opt
 		}
 
 		repo.IsMirror = true
+		if repo.Description == "" {
+			repo.Description = opts.Description
+		}
 		err = models.UpdateRepository(repo, false)
 	} else {
 		repo, err = CleanUpMigrateInfo(repo)