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.
Could you update this real quick to have dirstate_data_files | wc -l before this line? To make your comment in D12592 accurate.