Update object repo with the migrated repository (#18684)

When migrating a repository (from GitHub) using the API (**POST** `repos/migrate`), the Code Indexer is not updated. Searching in the user interface will not return any results.

When migrating the same repository using **+/New Migration** in the web interface, the search index is updated and searching works as expected.

Caused by the fact that object `repo` is never updated with the migrated repo so `setting.Indexer.RepoIndexerEnabled && !repo.IsEmpty` in `modules/notification/indexer/indexer.go:NotifyMigrateRepository` always evaluates to `false`.

Tested with gitea:1.16.1, MariaDB:10, Breve in `Run Mode: Dev`.

routers/api/v1/repo/migrate.go

@@ -203,7 +203,7 @@ func Migrate(ctx *context.APIContext) {
 		}
 	}()
 
-	if _, err = migrations.MigrateRepository(graceful.GetManager().HammerContext(), ctx.User, repoOwner.Name, opts, nil); err != nil {
+	if repo, err = migrations.MigrateRepository(graceful.GetManager().HammerContext(), ctx.User, repoOwner.Name, opts, nil); err != nil {
 		handleMigrateError(ctx, repoOwner, remoteAddr, err)
 		return
 	}