archive: preserve pre-open original atime, fixes #6194

[ThomasWaldmann]

When creating an archive with --atime on platforms without O_NOATIME, opening a file for reading could update atime before we recorded it (thus we archived the updated atime, not the original one).

Capture pre-open path-based-stat atime and use it if it pre-dates the atime we got from the fd AND if it refers to same fs object (avoid race condition).

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 81.09%. Comparing base (15f5923) to head (ec0cb41). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9180   +/-   ##
=======================================
  Coverage   81.08%   81.09%           
=======================================
  Files          78       78           
  Lines       13659    13663    +4     
  Branches     2043     2044    +1     
=======================================
+ Hits        11076    11080    +4     
  Misses       1910     1910           
  Partials      673      673           

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[ThomasWaldmann]

difficult to test in master: windows is too broken, lots of fail in cygwin.