rust-dirstate-v2: show `unused_bytes` counter is not reset on total rewrite This was picked up by @aalekseyev when doing unrelated debugging. The Rust implementation was never resetting this counter, so a brand new file would carry over the old counter. As I write this, my counter is a supposed 7389089 unused bytes for a total of 170978 bytes in the data file. Feel free to post your own high score. Differential Revision: https://phab.mercurial-scm.org/D12593

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