pg_upgrade needs postmaster [sic]

Hi,

I've played with trying to find out which minimal set of files I need
from the old version to make pg_upgrade work. Interestingly, this
includes the good old postmaster binary:

$ sudo -u postgres pgsql/bin/pg_upgrade -b /var/tmp/pgsql/bin/ -B /usr/lib/postgresql/9.5/bin/ -d /etc/postgresql/9.5/main -D /tmp/9.5/data
Finding the real data directory for the old cluster sh: 1: /var/tmp/pgsql/bin/postmaster: not found

Could not get data directory using "/var/tmp/pgsql/bin/postmaster" -D "/etc/postgresql/9.5/main" -C data_directory: No such file or directory
Failure, exiting

I think it should just use "postgres" there, patch attached. (If we
need to be compatible with postmaster-only PG versions from the last
century, it should try both names.)

Christoph
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