When initialising a manifest, use the ostree commit source spec only.
Then, when serialising, use the resolved ostree commit.
This is the same process we use for the other content types, packages
and containers.
Two new functions are defined in the distros to handle resolving the
ostree options: one for creating the parent commit source spec and image
ref for image types that have an optional parent commit (ostree commits
and containers) and one for creating the payload commit source spec for
image types that require an ostree payload (ostree raw images and
installers).
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