Fix bogus logic for checking data dirs' versions within pg_upgrade.

Commit 9be95ef1 failed to cure all of the redundancy here: we were
actually calling get_major_server_version() three times for each
of the old and new data directories.  While that's not enormously
expensive, it's still sloppy.

A. Akenteva

Discussion: https://postgr.es/m/f9266a85d918a3cf3a386b5148aee666@postgrespro.ru