rpm: generate changelog if none is provided

[UiP9AV6Y]

rpmlint considers a lack of a changelog an error.

this is a similar behaviour to the DEB packager, where a generic default changelog is generated unless one is provided explicitly.

[jordansissel]

Thank you for improving fpm! I especially appreciate the extra effort to add tests to cover this. Tests are passing and I checked manually:

% bundle exec bin/fpm -s empty -t rpm -n example
Created package {:path=>"example-1.0-1.noarch.rpm"}

% rpm -qp example-1.0-1.noarch.rpm --changelog
* Sat Dec 07 2024 <jls@baguette.local> - 1.0-1
- Package created with FPM