[ThomasWaldmann]

Backport of #10111 to 1.4-maint, fixes #6194.

borg has to open() a fs item before it can fstat() it. On platforms without O_NOATIME
support (or when we are not allowed to use it), that open() may already have updated the
atime - and then we archive an atime that we caused ourselves, instead of the one the item
had before borg touched it.

stat_update_check() now returns a os.stat_result look-alike (StatOrigAtime) that has
everything from the fd-based stat, but the atime from the stat done before opening the item.
This is only done if the atime actually changed between the two stat calls; otherwise the
plain stat result is returned as before.

2nd commit: test_atime no longer creates the hardlink (it shares the inode with input/file1,
which made the test fail on cygwin).

Differences from the master PR (1.4 test layout):

• the archiver tests are ArchiverTestCase methods in testsuite/archiver.py;
  test_atime_open_updates_atime is skipped in ArchiverTestCaseBinary (it patches objects).
• unit tests for stat_update_check() are in testsuite/archive.py, checking st_birthtime
  instead of st_birthtime_ns (not present on older Pythons).

Testing (macOS, py3.14): the new tests fail without the fix and pass with it. Full suite:
1468 passed, 382 skipped, 2 failed (test_unix_socket: "AF_UNIX path too long", caused by the
long tmp path of the local test setup, unrelated).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.19%. Comparing base (69552c0) to head (a70ccd1).
⚠️ Report is 7 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10577      +/-   ##
=============================================
+ Coverage      82.07%   82.19%   +0.12%     
=============================================
  Files             38       38              
  Lines          11485    11524      +39     
  Branches        1810     1817       +7     
=============================================
+ Hits            9426     9472      +46     
+ Misses          1470     1467       -3     
+ Partials         589      585       -4     

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