rust-dirstatemap: don't read the dirstate when requesting parents A future patch for issue 6303 reveals a big performance regression in the Rust `DirstateMap` that reads the entire dirstate when requesting parents instead of the first 40 bytes. `perfdiscovery` gets a *significant* speedup (from 0.101 to 0.016) when applied against said patch. I'm assuming it has other performance benefits, but this is already a good enough win. Differential Revision: https://phab.mercurial-scm.org/D8513

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