wrong parsing of SQL comments in initial SQL data

[samlowry]

Function custom_sql_for_model in core/management/sql.py incorrectly cut SQL comments - without considering to a query context. For example, function cuts end of such query:

INSERT INTO `table` (`description`) VALUES ('Come play on our boat -- On the Water');

It's because of a very simple regexp at line 187:

statement = re.sub(ur"^--.*([\n\Z]|$)", "", statement)

[Alex Gaynor]

Dupe of #3214.