Use the commit sources provided by the content on the initialised
manifest instead of the image options to resolve commits. The ostree
sources on the manifest are a map keyed by the name of the pipeline that
will use the resolved commit spec, but unlike with the package sets, the
worker's commit resolve job works on a slice, so we reread the content
map to get the pipeline name. This requires that there be only one
pipeline that requires a resolved ostree commit, which is currently true
of all our image types.
Setting the default ostree ref on the image options before calling
Manifest() isn't needed anymore.
962399d18a