tests: fix inode-order dependence in test_multiple_link_exclusion, fixes #8740

[ThomasWaldmann]

Fixes #8740.

borg create processes directory entries in inode order (scandir_inorder), so which of the two hardlinks becomes the hardlink master depends on the inode numbers the filesystem assigned to the dirs a and b. On linux ext4 these follow creation order, but e.g. FreeBSD ufs dirpref spreads new directories over cylinder groups, so b may get a lower inode than a — then b/hardlink became the master (with chunks) and diff restricted to input/b compared it successfully and printed just the ctime change (rc 0, exactly the output captured in the issue), instead of warning about the excluded hardlink source (rc 1). Same class of behavior on OpenIndiana.

The fix determines master/slave from the actual dir inode numbers, unlinks the master and restricts the diff to the slave's dir, so the test exercises the intended "hardlink source excluded" warning on any filesystem. Verified locally by simulating the FreeBSD inode order (creating b before a): the unfixed test fails with AssertionError: 0 != 1 like on FreeBSD, the fixed test passes in both orders.

Also drops the FreeBSD/NetBSD skip on this test: its failure was traversal-order dependence, not the kernel ctime quirks of #9147 / #9153 (which concern test_hard_link_deletion_and_replacement).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.11%. Comparing base (1b7d327) to head (17412cf).
⚠️ Report is 29 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10152      +/-   ##
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- Coverage      82.62%   82.11%   -0.52%     
=============================================
  Files             38       38              
  Lines          11432    11435       +3     
  Branches        1803     1802       -1     
=============================================
- Hits            9446     9390      -56     
- Misses          1408     1460      +52     
- Partials         578      585       +7     

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