This takes a list of refs + remotes and optional commit, all
which need to be p2p (i.e. have collection-id != NULL) refs, and
uses the p2p API to resolve the refs to the latest available commit
it and the matching metadata for that version.
It does this by doing a find_remotes() and then a pull_from_remotes
with PULL_FLAG_COMMIT_ONLY and then extracting the metadata
from the commit object.
We also do some checking before pulling so that if we have the latest
reported commit already available locally then we don't pull anything
for that ref (instead resolving to the locally available metadata).
We always pull to a child repo so that we have write-rights even as a
user (in the system case) and so we can properly clean up the temporary
results.
Note, this unconditionally uses the p2p APIs, and it relies on the latest
ostree master which has a fix that allows us to read the latest refs from
the transaction.
Closes: #1796
Approved by: alexlarsson
c647c315ef