replace InvalidXid(a macro that doesn't exist) with InvalidTransactionId(a macro that exists) in code comments

[Bharath Rupireddy]

Hi,

It seems like we have macro InvalidTransactionId but InvalidXid is
used in some of the code comments.Here's a small patch that does
$subject?

Regards,
Bharath Rupireddy.

v1-0001-replace-InvalidXid-with-InvalidTransactionId-in-c.patch
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