Don't overwrite PORTREVISION from the slave, following readline update.
I've had to bump revision for several slaves here, but most will not
be rebuilt, except the -client slaves. Apologies for anyone having
to rebuild -clients unnecessarily, but it's not a heavy task- better safe
than sorry.
PR: ports/236156
Reported by: Andrew Dunstan (PostgreSQL), koobs, Dmitri Goutnik