The core purpose of `Prepare_config()` is to prepare the config by setting
defaults where not specified. It should not be involved itself in
validation of user options.
This can be an important distinction, for instance in `clean` where we
use this function, but avoid validation to potentially allow for an option
that deletes the config, that should work even in the face of an invalid
config.
Gbp-Dch: Ignore
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