BUG #16783: pg_rewind: option -c does not work when configuration files are outside data directory

The following bug has been logged on the website:

Bug reference: 16783
Logged by: Edwin Polkerman
Email address: edwin(dot)polkerman(at)splendiddata(dot)com
PostgreSQL version: 13.1
Operating system: Centos7/8, RHEL7/8, SUSE12/15
Description:

To my opinion this report is more an improvement then a real bug.

We use PostgreSQL in such way that the configuration files are outside the
data directory. So for target location following applies:
- data location : /var/postgres/13/data
- config files location: /etc/postgres/13/data

On the target location 'restore_command' is filled in postgresql.conf.
Trying pg_rewind with following command:
pg_rewind --target-pgdata=/var/postgres/13/data -P -R -c
--source-server='host=<ip-address> port=5432 user=<user> password=<passwd>
dbname=postgres'

Following Fatal message is shown:
pg_rewind: fatal: restore_command is not set in the target cluster

pg_rewind looks at the postgresql.conf in /var/postgres/13/data in stead of
the location where the configuration files reside (/etc/postgres/13/data).
It would be nice to add an option to tell pg_rewind that configuration files
are on a different location.