import-tar: fix nfiles being counted twice for regular files

[ThomasWaldmann]

Summary

_import_tar incremented archive.stats.nfiles for every regular tar member, but TarfileObjectProcessors.process_file already counts those in tfo.stats (and process_hardlink counts hardlink members). Since archive.stats += tfo.stats is added before archive.save(), imported archives persisted nfiles = 2 * regular_files + hardlinks into the archive metadata, so borg info / borg repo-list reported a wrong file count for imported archives.

Remove the redundant increment — borg create also relies solely on the processor-side accounting, so import-tar now matches its semantics (each regular file and each hardlink counts once; directories/symlinks/devices/fifos do not).

Test

Added test_import_tar_nfiles: builds a tar via Python's tarfile module (no GNU tar or filesystem hardlink support needed) containing 2 regular files, 1 hardlink member, 1 directory and 1 symlink, imports it, and asserts the archive metadata reports nfiles == 3. Verified the test fails with assert 5 == 3 on the unfixed code.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.95%. Comparing base (9547a53) to head (30194fb).
⚠️ Report is 33 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10146   +/-   ##
=======================================
  Coverage   86.94%   86.95%           
=======================================
  Files         101      101           
  Lines       17984    17983    -1     
  Branches     2737     2737           
=======================================
  Hits        15637    15637           
- Misses       1638     1639    +1     
+ Partials      709      707    -2     

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[github-actions]

Backport failed for 1.4-maint, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 1.4-maint
git worktree add -d .worktree/backport-10146-to-1.4-maint origin/1.4-maint
cd .worktree/backport-10146-to-1.4-maint
git switch --create backport-10146-to-1.4-maint
git cherry-pick -x 30194fb64f747c0b78900149687d78c213f08b8c