Reset stale local mirror cache when remote is empty

[matt8s]

This fixes a push failure when the rclone remote has no repository archive/refs but the helper still has an old local mirror under .git/rclone/<remote>/repo.

In that state, a push of unrelated history can fail in git fast-import with a non-fast-forward warning because the stale mirror constrains the new branch even though the actual remote is empty.

The change:

• treats a remote with no repo archive and no refs as genuinely empty;
• clears the helper's local mirror/marks cache before exporting to it;
• preserves the existing recovery behavior when the archive disappears but refs still remain;
• adds a regression test that creates a remote, pushes once, deletes the remote contents, replaces the local branch with unrelated history, and pushes again.

I encountered this on RHEL 9 after reusing a local repository whose helper cache contained history from an earlier remote state. Moving .git/rclone/origin aside allowed the push to succeed, which led to this fix.

This is intentionally separate from #7 and is based directly on current main, so the two PRs can be reviewed and merged independently.