GeezerVison

Fixes #360

-N/--new-file (and --unidirectional-new-file for the first operand) turned every missing operand into an empty file without looking at the other operand. So diff -N missing1 missing2 compared two empty files, printed nothing and exited 0, and a typo in both paths went unnoticed.

GNU diff only treats a missing top-level operand as empty when the other one exists. compare_files now reads both operands first, and if both are missing it reports both errors and returns trouble (exit 2), like GNU:

$ diff -N missing1 missing2
diff: missing1: No such file or directory
diff: missing2: No such file or directory
$ echo $?
2

When only one operand is missing, the behaviour is the same as before.

Test: new_file_both_missing_is_an_error in tests/by-util/test_diff.rs covers -N, --new-file and --unidirectional-new-file. It fails on main and passes with this change. cargo test --test tests passes (43 tests), and cargo fmt --check and cargo clippy -- -D warnings are clean. The 16 *_permutations lib tests fail on my machine on main too, so that failure isn't caused by this change.

I used an AI assistant to write this change. I reviewed and tested it myself.