Fixed #23303 -- Add BEGIN and COMMIT statements to the output of sqlmigr...

...ate.
I've added the blank line after `BEGIN` for readability since the rest of the SQL is heavily reformatted anyway.

For reference, the blank line before `COMMIT` is caused by this: https://github.com/django/django/blob/master/django/core/management/base.py#L424.
I'm not quite sure why it's there but I'm wary of changing this now (though removing the newline doesn't seem to break any tests).