Fixed #29295 -- Fixed management command crash when using subparsers.

Thanks Tim Graham for the fix.

tests/user_commands/management/commands/subparser.py

@@ -0,0 +1,12 @@
+from django.core.management.base import BaseCommand
+
+
+class Command(BaseCommand):
+
+    def add_arguments(self, parser):
+        subparsers = parser.add_subparsers()
+        parser_foo = subparsers.add_parser('foo')
+        parser_foo.add_argument('bar', type=int)
+
+    def handle(self, *args, **options):
+        self.stdout.write(','.join(options))