pg_upgrade: Preserve default char signedness value from old cluster.
Commit
44fe30fdab6 introduced the 'default_char_signedness' field in
controlfile. Newly created database clusters always set this field to
'signed'.
This change ensures that pg_upgrade updates the
'default_char_signedness' to 'unsigned' if the source database cluster
has signedness=false. For source clusters from v17 or earlier, which
lack the 'default_char_signedness' information, pg_upgrade assumes the
source cluster was initialized on the same platform where pg_upgrade
is running. It then sets the 'default_char_signedness' value according
to the current platform's default character signedness.
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/
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