loader: Fix error when first revision number is not one
It exist cases where the first revision number returned by a subversion server has a value greater than one.
The loader did not expect such cases in its current implementation and was raising an error preventing the loading of such repository in the archive (apart when using the SvnLoaderFromRemoteDump class which always dumps revisions starting from number one).
So ensure to cover that case in loader implementation.
Related to #3701 (closed)
Migrated from D6602 (view on Phabricator)