cmp: use .map_err

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will degrade performance by 5.46%

⚠️ 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

❌ 1 regressed benchmark
✅ 25 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cmp_parser	5.6 µs	5.9 µs	-5.46%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:cmp-map_err (5740d85) with main (f29e96c)}

[github-actions]

GNU diffutils testsuite comparison:

Test results comparison:
  Current:   TOTAL: 33 / PASSED: 0 / FAILED: 33 / SKIPPED: 0
  Reference: TOTAL: 33 / PASSED: 8 / FAILED: 21 / SKIPPED: 4

Changes from main branch:
  TOTAL: +0
  PASSED: -8
  FAILED: +12

New test failures (12):
  - basic
  - bignum
  - brief-vs-stat-zero-kernel-lies
  - bug-64316
  - cmp
  - diff3
  - help-version
  - large-subopt
  - strcoll-0-names
  - strip-trailing-cr
  - timezone
  - y2038-vs-32bit

[marc-hb]

Need more coffee? Sincere apologies for the noise.

[oech3]

No problem. Thankyou for reviewing.

[cakebaker]

Thanks!