Ironically this function has a bug:

Basically it should not reuse the savepoint name sp.

If you do something like:

BEGIN;
INSERT INTO t VALUES (1);
SAVEPOINT sp;
INSERT INTO t VALUES (2);
SAVEPOINT sp;
INSERT INTO t VALUES (3);
ROLLBACK TO sp;
ROLLBACK TO sp;
COMMIT;

then the table will contain 1 and 2!

And while you probably won't run into this in most code,
it can happen much easier if you have code generic over
GenericConnection which uses a transaction.

Possible fixes:

1. always generate a name including the
   current depth (format!("sp{}", self.depth))
2. only generate a name if the depth > 1
   (assuming that it starts with 0 on first trans without savepoint, 1 on second etc.)

When we are at it we might want to change savepoint
to accept <S: Into<String>> instead of &str.

I will see if I can make a pull request, but I won't
have time for it before the weekend.