Fix GetOldestXmin comment

Hi,

While reading source code, I realized that comment of GetOldestXmin mentions;

* if rel = NULL and there are no transactions running in the current
* database, GetOldestXmin() returns latestCompletedXid.

However, in that case if I understand correctly GetOldestXmin()
actually returns latestCompletedXid + 1 as follows;

/*
* We initialize the MIN() calculation with latestCompletedXid + 1. This
* is a lower bound for the XIDs that might appear in the ProcArray later,
* and so protects us against overestimating the result due to future
* additions.
*/
result = ShmemVariableCache->latestCompletedXid;
Assert(TransactionIdIsNormal(result));
TransactionIdAdvance(result);

Attached patch fixes the top comment of GetOldestXmin.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center