For now this is simply used to resolve the parent commit, in case
one is not provided. In the future it will be used by new image
types to actually pull content from.
This extends the weldr API, so that future work does not have to
modify that.
The logic we now implement for the ostree commit image types is:
If the URL is provided, but the parent commit is not. The parent
commit is taken to be the current HEAD of the ostree repo at the
given url, with the given (or default) ref.
This only provides a small optional convenience, but we will
soon introduce image types where the URL of the repository is
required.
This commit still needs testing.
Signed-off-by: Tom Gundersen <teg@jklm.no>
687ac7f615