Add mercurial directory loader
This provides the class HgDirectoryLoader with only the fetch_artifact
method.
This clones a repository at a specific changeset (or tag) and ingests the DAG objects from the
resulting directory tree. It also checks the checksums (Nar or standard) if provided.
The Nar checksums are stored in the extid mapping table when provided (when checks are fine).
Refs. swh/meta#4979 (closed)
Edited by Antoine R. Dumont