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