date: report invalid UTF-8 lines in -f file/stdin input

[alexchen-sys]

Problem

date -f reads lines via BufRead::lines().map_while(Result::ok). On a line that is not valid UTF-8, lines() yields an InvalidData error and map_while silently ends the iterator: the offending line and everything after it vanish with no diagnostic, and date exits 0 — even with --debug, which stays silent because it never sees the line.

GNU date (current git master) instead reports the line (non-printable bytes octal-escaped), keeps processing the rest of the input and exits 1:

$ printf 'Hello\xffx\n' > bad.txt && date -f bad.txt
date: invalid date 'Hello\377x'

Fix

Read the input with split(b'\n'), stripping a trailing \r, and decode each chunk as UTF-8. A chunk that fails to decode becomes an invalid-date error via the existing escape_invalid_bytes helper (already used for the same diagnostic for the -d argument), producing the GNU-style invalid date 'Hello\377x' message; iteration continues with the remaining lines and the final exit status is 1, matching GNU.

Tests

Three new tests in tests/by-util/test_date.rs:

• a file with a single invalid-UTF-8 line fails with code 1 and the GNU-shaped message on stderr
• a mixed file still prints the valid dates around the bad line while failing with code 1
• the same input through stdin (-f -) behaves identically

Verified against a date binary built from GNU git master (aea70b2): same diagnostic (byte-for-byte in the C locale — GNU renders typographic quotes under UTF-8 locales), same exit code, remaining lines still processed.

Fixes #12920

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.18 MB (+1004 KB, +0.65%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!