create: do not archive an atime we caused ourselves, fixes #6194

[ThomasWaldmann]

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 we did before opening the
item - as suggested in the issue. This is only done if the atime actually changed between
the two stat calls, so when O_NOATIME worked, the plain stat result is returned as before
(no proxy object, no overhead).

Doing this in stat_update_check() covers all its call sites at once: regular files, fifos,
devices and both directory opens.

Note: this only changes what we archive - without O_NOATIME we still can not avoid
updating the atime in the source filesystem.

Tests:

• test_atime_open_updates_atime simulates a platform where the open() already updates the
  atime (by reading a byte in a monkeypatched os_open) and checks that the archived atime is
  still the one from before. It fails without the fix.
• unit tests for stat_update_check(): atime fixup, unchanged atime (returns the stat result
  as is) and the two race condition checks, which had no test coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.07%. Comparing base (daa1944) to head (9c3a5ef).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10111      +/-   ##
==========================================
+ Coverage   87.03%   87.07%   +0.04%     
==========================================
  Files         101      101              
  Lines       17730    17740      +10     
  Branches     2678     2679       +1     
==========================================
+ Hits        15431    15447      +16     
+ Misses       1598     1595       -3     
+ Partials      701      698       -3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.