Right now, we first load the packed refs cache and only afterwards load the
loose references. This is susceptible to a race when the loose ref is being
migrated to a packed cache by e.g. git-pack-refs(1):

                libgit2                             git-pack-refs

   1. We load the packed ref, which
      does not yet have the migrated
      reference.

                                      2. git-pack-refs updates the packed ref
                                         file to have the migrated ref.

                                      3. git-pack-refs deletes the old loose
                                         ref.

   4. We look up the loose ref.

So we now do not find the reference at all and will never iterate over it.

Fix the issue by reversing the order: instead of first loading the packed refs,
we will now look up the loose reference first. If it has already been deleted,
then it must already be present in the packed-refs by definition, as git.git
will only delete the reference after updating the packed refs file.