find: share one file handle per output path (fixes #439)

[MsfPablo]

Fixes #439.

Problem

GNU find de-duplicates the files opened by its output predicates, so
find . -fprint foo -fprint foo writes each path to foo twice. uutils
opened a separate File for every occurrence of -fprint/-fprintf/
-fprint0/-fls, so each handle had its own file offset and the writes
overwrote one another — only one line per entry survived.

Fix

get_or_create_file now caches opened output files on Config, keyed by the
path as written on the command line (matching GNU's literal-filename
de-duplication), and hands out a shared Rc<File>. Repeated references to the
same path therefore share a single file descriptor and a single file offset.
Printer, Printf and Ls take Option<Rc<File>> instead of Option<File>.
The first reference still truncates, as before.

Tests

Two unit tests in src/find/matchers/mod.rs:

• get_or_create_file_reuses_handle_for_same_path — the same path yields the
  same Rc, and writes through both handles append rather than overwrite.
• two_fprints_to_the_same_file_do_not_overwrite_each_other — end-to-end
  through build_top_level_matcher with -fprint out -fprint out, asserting
  the entry appears twice.

Verified the regression test fails without the fix (reverting just the cache
lookup gives left: "…abbbc\n…abbbc\n" / right: "…abbbc\n").

$ cargo test
test result: ok. 228 passed; 0 failed  (lib)
... all other suites ok, 0 failed
$ cargo clippy --all-targets --all-features -- -D warnings
    Finished `dev` profile
$ cargo fmt --all --check
(clean)

AI disclosure

This change was written with the assistance of Claude (Anthropic). I have
reviewed the diff, and ran the build, the full test suite, clippy and rustfmt
locally as shown above.

[github-actions]

Commit 2081a60 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 313 / PASSED: 266 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +0
  PASSED: -1
  FAILED: +1

New test failures (1):
  - gnu/okdir_path_relative

[sylvestre]

many jobs are failing

[sylvestre]

could you please fix the conflict? thanks

[github-actions]

Commit 2289f66 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6
  Reference: TOTAL: 314 / PASSED: 266 / FAILED: 42 / SKIPPED: 6

Changes from main branch:
  TOTAL: +3
  PASSED: +8
  FAILED: -5

Test improvements (5):
  + gnu/execdir_path_dot
  + gnu/execdir_path_empty
  + gnu/execdir_path_relative
  + gnu/files0_from_ok
  + gnu/okdir_path_empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing MsfPablo:fix-439-dedup-output-files (2b5c84f) with main (1acf40a)}

[github-actions]

Commit 2b5c84f has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 421 / FAILED: 73 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 420 / FAILED: 74 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + size-invalid.new-O3

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (1acf40a) to head (2b5c84f).

Files with missing lines	Patch %	Lines
src/find/matchers/ls.rs	50.00%	1 Missing ⚠️
src/find/matchers/mod.rs	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #828      +/-   ##
==========================================
+ Coverage   92.22%   92.26%   +0.03%     
==========================================
  Files          35       35              
  Lines        7435     7471      +36     
  Branches      386      387       +1     
==========================================
+ Hits         6857     6893      +36     
  Misses        437      437              
  Partials      141      141              

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