loader: Optimize SvnLoaderFromRemoteDump use on stale repository
When trying to load multiple times a stale subversion repository with SvnLoaderFromRemoteDump, first check if the last loaded revision in the archive is different from the one on the remote subversion server.
If they are identical skip the dump, mount and load phases in order to gain some disk space and processing time on the celery worker executing the loading task.
Related to #3719
Migrated from D6658 (view on Phabricator)