bitbucket: Allow incremental lister to start properly the first time

When there is no data in db, prior to this commit, it insisted on listing starting from around now. In the end, listed only a few repositories.

This reduces the scope of this diff to this commit only

Related #1826 (closed) Depends on D1635

Test Plan

tox

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Migrated from D1632 (view on Phabricator)