GeezerVison

Fixes #361

With -N / --new-file, a missing operand is treated as empty. GNU dates it at the Unix Epoch (1970-01-01 00:00:00) in the unified and context headers. That is the convention patch uses to tell that a file must be created or removed. get_modification_time fell back to the current time for any metadata error, so the missing side got today's date and patch left an empty file behind instead of deleting it:

$ TZ=UTC0 diff -N -u A/only B/only
--- A/only	2026-11-09 15:18:23.146917271 +0000
+++ B/only	2026-11-09 15:18:23.167832210 +0000   # GNU: 1970-01-01 00:00:00.000000000 +0000

get_modification_time now returns the Epoch when the file does not exist (ErrorKind::NotFound). Standard input (-) and other errors keep the current time as before. This covers -u, -c (both timestamp styles) and -r -N directory comparisons, since they all go through this function. Output now matches GNU diff 3.x on the box for all three.

Tests:

• tests/by-util/test_diff.rs: new_file_missing_side_dated_at_epoch (unix only, since it sets TZ like context_timestamp_follows_time_locale) checks -u, -c with an ISO locale and -c with LC_ALL=C, with the missing file on either side, plus -r -N -u over two directories. It fails on main and passes with this change.
• src/utils.rs: the invalid_file unit test asserted the old "now" fallback for a missing file. It is replaced by missing_file (expects the Epoch) and stdin (expects the current time for -).

cargo test --test tests (43 passed), cargo test --lib modification_time, cargo fmt --check and cargo clippy -- -D warnings are clean.

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