Add parameter 'ignore_unresolved' to snapshot_git_object
It allows building a snapshot manifest despite some branches being unresolved (instead of raising an error).
This feature was removed in !302 (closed) but in the end, Snapshot.compute_hash() will need to use it so that swh-scrubber does not crash when checksum such snapshots.
Migrated from D8201 (view on Phabricator)