readOneRecord() is used now when looking for a checkpoint record to
check if the target server is an ancestor of the source across multiple
timelines, and using a restore_command if available improves the
stability of the operation.  This part was missed in a7e8ece4.

Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20200421.150830.1410714948345179794.horikyota.ntt@gmail.com