pg_upgrade: retrieve subscription count more efficiently

When Who What
2024-07-24 16:34:06 Nathan Bossart (bossartn) Closed in commitfest 2024-09 with status: Committed
2024-07-24 16:34:06 Nathan Bossart (bossartn) Changed committer to bossartn
2024-07-21 02:03:58 Nathan Bossart (bossartn) Changed authors to Nathan Bossart (bossartn)
2024-07-21 02:03:58 Nathan Bossart (bossartn) Changed targetversion to 17
2024-07-21 02:03:52 Nathan Bossart (bossartn) Attached mail thread 20230217075433.u5mjly4d5cr4hcfe@jrouhaud
2024-07-21 02:03:51 Nathan Bossart (bossartn) Created patch record