utils: Ensure to return correct revision in parse_external_definition
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
Migrated from D8003 (view on Phabricator)