Refactor setting XLP_FIRST_IS_OVERWRITE_CONTRECORD.
Set it directly in CreateOverwriteContrecordRecord(). That way,
AdvanceXLInsertBuffer() doesn't need the missingContrecPtr global
variable. This is in preparation for splitting xlog.c into multiple
files.
Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/
a462d79c-cb5a-47cc-e9ac-
616b5003965f%40iki.fi