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

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Migrated from D8003 (view on Phabricator)