according to the standard, the gtrid/bqual pair must be
"globally unique", so XIDs are equal if their gtrids and
bquals are identical, formatID simply tells how
gtrid/bqual were generated. But if formatID == -1
(it means "null") gtrid/bqual don't have any value at all.

This fixes main.xa_sync failure in
./mtr main.quary_cache_debug main.xa_sync