diff: accept clustered short options such as -qst and -ut

[SichenLiang]

GNU diff allows short options that take no argument to be combined, so diff -qst a b is the same as diff -q -s -t a b. uutils treated the combined form as one unknown option.

$ printf 'a\nb\n' > a; printf 'a\nc\n' > b
$ diff -qst a b; echo $?
Files a and b differ
1

The existing argument-free options (-c, -e, -q, -s, -t, -u and -y) now use the same path whether they are separate or clustered. Output-style conflicts are checked in the same place, while a word containing an unsupported option keeps the old handling.

Fixes #298

[codspeed]

Merging this PR will improve performance by 3.15%

⚠️ 24 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• cmd_cmp_gnu_equal[25000]
• cmd_cmp_gnu_equal[10000]
• cmd_diff_gnu_equal[10000]
• cmd_diff_gnu_equal[1000]
• cmd_diff_gnu_equal[25000]
• cmd_cmp_gnu_equal[100]
• cmd_diff_gnu_equal[100]
• cmd_diff_release_equal[1000]
• cmd_cmp_gnu_equal[1000]
• cmd_diff_release_equal[25000]
• …and 14 more

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 8 improved benchmarks
✅ 18 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡ ⚠️	cmd_cmp_gnu_equal[25000]	23 µs	22.3 µs	+3.22%
⚡ ⚠️	cmd_cmp_gnu_equal[10000]	23 µs	22.3 µs	+3.21%
⚡ ⚠️	cmd_diff_gnu_equal[10000]	23.1 µs	22.3 µs	+3.21%
⚡ ⚠️	cmd_diff_gnu_equal[1000]	23.1 µs	22.3 µs	+3.21%
⚡ ⚠️	cmd_diff_gnu_equal[25000]	23.1 µs	22.4 µs	+3.2%
⚡ ⚠️	cmd_cmp_gnu_equal[100]	23.6 µs	22.9 µs	+3.13%
⚡ ⚠️	cmd_diff_gnu_equal[100]	24.4 µs	23.7 µs	+3.03%
⚡ ⚠️	cmd_diff_release_equal[1000]	24.4 µs	23.6 µs	+3.03%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing SichenLiang:short-option-clusters (385935e) with main (2d605f6)}