loader: Simplify the handing of parent revision

In the current loader implementation, a revision different from the first one has a single parent revision corresponding to the previously processed one.

In order to simplify the handling of the parent revision, remove the use of the revision_parents dict and simply store the previously processed revision id in a parents tuple variable while iterating on the revisions log. In case of incremental loading, that tuple will be initialized from the latest revision loaded into the archive.

This change is required to allow the loading of svn subprojects.

Related to #3896 (closed)

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Migrated from D7137 (view on Phabricator)