git loader: add an option to force re-download/ingest everything

By default the Git loader only download objects that are determined to be missing from the archive, relying on Merkle structure properties. It is however useful to force re-downloading everything, e.g., to re do past ingestions that didn't work for whatever reasons (e.g., bugs in the loader).

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Migrated from T1212 (view on Phabricator)