fix off-by-1us timestamp in revisions coming from SVN loader

As part of investigations for swh/devel/experiments/swh-db-audit#75 (moved), @olasd noticed that revisions being added by the SVN loader (currently running on Google Code SVN dumps) reach the Postgres DB with an off-by-1us (microsecond) timestamp, resulting in additional inconsistent checksums.

According to early investigations this happens //after// passing through our model module, on the way to the database.

This should be fixed ASAP, and certainly before restarting the SVN loader tasks

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Migrated from T680 (view on Phabricator)