loader: Do not raise error when reloading a repo not fully ingested
When a subversion repository did not get fully ingested during a first load (an error occurred when processing a revision that stopped the loading), trying to reloading it again will raise a ValueError instead of generating the same snapshot as the previous visit.
So ensure to initialize the _snapshot and _last_revision class member of the loader to the latest loaded snapshot and revision if they exist to avoid raising an error and generate an uneventful visit.
Fixes SWH-LOADER-SVN-1P.
Migrated from D6778 (view on Phabricator)