Rename the `diffutils` binary to `diff`, #235

Job	Run time
cargo check (ubuntu-latest)	16s
cargo fmt --all -- --check	11s
cargo clippy -- -D warnings (ubuntu-latest)	17s
cargo check (macOS-latest)	21s
Code Coverage (ubuntu-latest, unix)	2m 21s
cargo clippy -- -D warnings (macOS-latest)	13s
cargo check (windows-latest)	1m 20s
Code Coverage (macos-latest, macos)	2m 39s
cargo clippy -- -D warnings (windows-latest)	2m 9s
GNU test suite	27s
cargo test (ubuntu-latest)	46s
Code Coverage (windows-latest, windows)	5m 54s
cargo test (macOS-latest)	3m 19s
cargo test (windows-latest)	4m 1s
	24m 14s

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