Revert patch for taking fewer snapshots.

This reverts commit d573e239f0, "Take fewer
snapshots".  While that seemed like a good idea at the time, it caused
execution to use a snapshot that had been acquired before locking any of
the tables mentioned in the query.  This created user-visible anomalies
that were not present in any prior release of Postgres, as reported by
Tomas Vondra.  While this whole area could do with a redesign (since there
are related cases that have anomalies anyway), it doesn't seem likely that
any future patch would be reasonably back-patchable; and we don't want 9.2
to exhibit a behavior that's subtly unlike either past or future releases.
Hence, revert to prior code while we rethink the problem.

src/backend/commands/prepare.c

@@ -289,7 +289,7 @@ ExecuteQuery(ExecuteStmt *stmt, IntoClause *intoClause,
 	/*
 	 * Run the portal as appropriate.
 	 */
-	PortalStart(portal, paramLI, eflags, true);
+	PortalStart(portal, paramLI, eflags, GetActiveSnapshot());
 
 	(void) PortalRun(portal, count, false, dest, dest, completionTag);