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.