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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complete hg \
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