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
Migrated from D1632 (view on Phabricator)