fix data race on migrate repository (#5224)

Author: lunny

models/repo.go

@@ -1043,7 +1043,6 @@ func MigrateRepository(doer, u *User, opts MigrateRepoOptions) (*Repository, err
 		if err = SyncReleasesWithTags(repo, gitRepo); err != nil {
 			log.Error(4, "Failed to synchronize tags to releases for repository: %v", err)
 		}
-		UpdateRepoIndexer(repo)
 	}
 
 	if err = repo.UpdateSize(); err != nil {
@@ -1061,10 +1060,16 @@ func MigrateRepository(doer, u *User, opts MigrateRepoOptions) (*Repository, err
 		}
 
 		repo.IsMirror = true
-		return repo, UpdateRepository(repo, false)
+		err = UpdateRepository(repo, false)
+	} else {
+		repo, err = CleanUpMigrateInfo(repo)
+	}
+
+	if err != nil && !repo.IsBare {
+		UpdateRepoIndexer(repo)
 	}
 
-	return CleanUpMigrateInfo(repo)
+	return repo, err
 }
 
 // cleanUpMigrateGitConfig removes mirror info which prevents "push --all".