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