truncate, paste, uniq: let a repeated option override the earlier one

[arbelonson-source]

GNU reads options left to right, so giving the same one twice keeps the last value. These three rejected the repeat outright.

Command	GNU	uutils before
truncate -s 1 -s 2 f	truncates to 2	error: the argument '--size <SIZE>' cannot be used multiple times
paste -d, -d: f1 f2	joins with :	error: the argument '--delimiters <LIST>' cannot be used multiple times
uniq -f 1 -f 2 f	skips 2 fields	error: the argument '--skip-fields <N>' cannot be used multiple times

Fixed with args_override_self(true), the same way cut already does it. For uniq this covers -s and -w as well, both checked against GNU.

Found by differential testing against GNU coreutils 9.11.

This is deliberately not applied everywhere. cut -f1 -f2 and join -1 1 -1 2 are errors in GNU too — just different ones ("only one list may be specified", "incompatible join fields 0, 1") — so those are left alone rather than swept up. cut in particular already documents in a comment why it uses ArgAction::Append to count occurrences.

Testing

One regression test per utility. Verified they catch the bug by reverting the three source files alone — all three fail, then pass with them restored.

• cargo test --features "truncate,paste,uniq" --no-default-features: 99 passed, 0 failed (96 pre-existing, 3 new)
• cargo fmt --check and cargo clippy -p uu_truncate -p uu_paste -p uu_uniq --all-targets: clean
• 9-case differential check covering the repeated forms plus the single-option ones (truncate -s 3, paste -d,, uniq -f 1, bare uniq) as regression cover: all 9 match GNU

Disclosure

Prepared with AI assistance (Claude Code), per the AI policy in CONTRIBUTING.md. On the GPL point raised there: expected behavior was established by running the installed GNU binaries as a black box and recording their output. I did not read GNU coreutils source. All testing above was run locally.

[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:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)