.arcconfig

unit-tests: Fix `cargo test` on 32-bit platforms

Fixes https://bz.mercurial-scm.org/show_bug.cgi?id=6506

This makes `IndexEntryBuilder::build`, which is only used in unit tests, use
`u32` or `u64` instead of platform-dependent `usize` when packing binary data
to be used at test input.

To run Rust unit tests in 32-bit mode in a x86-64 environment, use:

    rustup target add i686-unknown-linux-gnu  # Once
    (cd rust && cargo test --target i686-unknown-linux-gnu)

Differential Revision: https://phab.mercurial-scm.org/D10351

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