Query tool result discarding

Hi!

A fairly typical use-case for the query tool in my org is:

BEGIN TRANSACTION;
UPDATE something SET somecol=newval WHERE something=other;
SELECT * FROM something; -- this is to manually verify results
ROLLBACK;

Then when everything works, you replace ROLLBACK with COMMIT. This works
perfectly in the query tool in sqlserver, but pgAdmin discards the
result from SELECT and leaves the result pane empty. Because it's
overwritten by the empty result from ROLLBACK.

Attached patch fixes this very limited test case. I'm well enough
unfamiliar with that code that I think it's probably not the correct
solution "in the big scheme" (and it also seems too easy), but it might
help point one of you guys who know that code better in the right
direction. Or on the off chance that it's right, hey, there was some
luck left over :-)

//Magnus