Prevent duplicate labels when importing more than 99 (#22591)

Backport #22591

Importing labels (via `gitea restore-repo`) did not split them up into
batches properly. The first "batch" would create all labels, the second
"batch" would create all labels except those in the first "batch", etc.
This meant that when importing more than 99 labels (the batch size)
there would always be duplicate ones.

This is solved by actually passing `labels[:lbBatchSize]` to the
`CreateLabels()` function, instead of the entire list `labels`.

services/migrations/migrate.go

@@ -282,7 +282,7 @@ func migrateRepository(doer *user_model.User, downloader base.Downloader, upload
 				lbBatchSize = len(labels)
 			}
 
-			if err := uploader.CreateLabels(labels...); err != nil {
+			if err := uploader.CreateLabels(labels[:lbBatchSize]...); err != nil {
 				return err
 			}
 			labels = labels[lbBatchSize:]