loader: Ensure to fetch latest snapshot produced by a hg visit type
requested to merge anlambert/swh-loader-mercurial:snapshot_get_latest_visit_type_filtering into master
SWH data model allows an origin to have multiple visit types, in particular a mercurial origin can have visit types 'hg' and 'hg-checkout'.
We must ensure to retrieve the latest snapshot for a hg visit type in the mercurial loader implementation as it can break incremental loading of a mercurial origin having both visit types mentioned above.
Related to swh/meta#5092.