loader: Allow to load a repository with unresolved deltas in pack file
It exists some rare cases when a remote git repository sends a pack file where some deltas in it cannot be resolved, meaning some git objects end up missing in the local repository.
Previously the loader was failing when encountering such repositories but almost all of the git objects are available in the pack file so prefer to load them into the archive and set the visit status as partial instead.
Related to #4748.