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

 Fix bogus logic for checking data dirs' versions within pg_upgrade.
 
 Commit 9be95ef15 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