[PATCH 07/14] Introduce InvalidCommandId and declare that to be the new maximum for CommandCounterIncrement

This is useful to be able to represent a CommandId thats invalid. There was no
such value before.

This decreases the possible number of subtransactions by one which seems
unproblematic. Its also not a problem for pg_upgrade because cmin/cmax are
never looked at outside the context of their own transaction (spare timetravel
access, but thats new anyway).
---
 src/backend/access/transam/xact.c | 4 ++--
 src/include/c.h                   | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)