An external definition can be of the following form (where XXX and YYY
are revision numbers):
-r XXX <repo_url>@YYY
In that case, the official subversion client will export the revision
XXX of the external repository.
So ensure to have the same behavior when the subversion loader processes
a repository with such external defintion in it.
Related sentry issue: SWH-LOADER-SVN-2Y