git.loader: fix failing origin visit update step due to uninitialized internal state variables

Currently, a failure happens for some reason. As some internal state variables are not initialized (they are in fetch_data but not in __init__), this makes the get_snapshot method fails because of it. Then that's making the visit update step fails.

Adding the proper initialization for those variables in the __init__ method should fix it. That will allow the failed state of the visit to be updated properly in the storage. Associating an empty snapshot to the failed visit.

This should fix issues like [1]

• [1] https://sentry.softwareheritage.org/share/issue/ace9143c0f6244b49b18cca49a5aa809/

Test Plan

tox

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Migrated from D3019 (view on Phabricator)