Fixed object lookup

Signed-off-by: Enrico Zini <enrico@enricozini.org>

contributors/management/commands/usermerge.py

@@ -33,7 +33,7 @@ class Command(BaseCommand):
 
         author = User.objects.get(username=author)
         dest = User.objects.get(username=into)
-        src = [User.objects.get(x) for x in users]
+        src = [User.objects.get(username=x) for x in users]
 
         if dest in src:
             raise CommandError(f"Cannot merge User {src} into itself")