Do not assert on diff if hard link sources are not found due to exclusions

[cr1901]

Per IRC, borg diff will assert if a multiply-linked file is selected for compare, but its hard link source was not selected for compare. Print a warning and allow the diff to continue.

Example failure:

william@xubuntu-dtrain:/shares/wjones/work/browser-hist$ borg diff --content-only ::WD3200BEVT-userprofile-2021-10-17-120019 WD3200BEVT-userprofile-2021-10-24-120006 --pattern="+ pf:" --pattern="+ pp:Downloads" --pattern="! **/" > /dev/null
Local Exception
Traceback (most recent call last):
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 5412, in main
    exit_code = archiver.run(args)
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 5330, in run
    rc = func(args)
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 191, in wrapper
    return method(self, args, repository=repository, **kwargs)
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 206, in wrapper
    return method(self, args, repository=repository, manifest=manifest, key=key, archive=archive, **kwargs)
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 1172, in do_diff
    for path, diff in diffs:
  File "/home/william/Projects/borg/borg/src/borg/archiver.py", line 1167, in <genexpr>
    diffs = ((path, diff.changes()) for path, diff in diffs if not diff.equal)
  File "/home/william/Projects/borg/borg/src/borg/archive.py", line 1120, in compare_archives_iter
    assert hardlink_master_seen(item1)
AssertionError

Platform: Linux xubuntu-dtrain 5.15.0-112-generic #122-Ubuntu SMP Thu May 23 07:48:21 UTC 2024 x86_64
Linux: Unknown Linux
Borg: 1.4.1.dev26+gad502848  Python: CPython 3.10.12 msgpack: 1.0.8 fuse: pyfuse3 3.2.0 [pyfuse3,llfuse]
PID: 3004014  CWD: /shares/wjones/work/browser-hist
sys.argv: ['/home/william/.local/bin/borg', 'diff', '--content-only', '::WD3200BEVT-userprofile-2021-10-17-120019', 'WD3200BEVT-userprofile-2021-10-24-120006', '--pattern=+ pf:', '--pattern=+ pp:Downloads', '--pattern=! **/']
SSH_ORIGINAL_COMMAND: None

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.51%. Comparing base (938e144) to head (2720331).
Report is 14 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/archive.py	66.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8344      +/-   ##
=============================================
- Coverage      82.73%   82.51%   -0.23%     
=============================================
  Files             38       38              
  Lines          11049    11081      +32     
  Branches        2123     2133      +10     
=============================================
+ Hits            9141     9143       +2     
- Misses          1344     1369      +25     
- Partials         564      569       +5     

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[ThomasWaldmann]

@cr1901 Is this still on your radar?

[cr1901]

Yes, it's still on my radar. Though, I've been working on trying to finish several things this past month, and am coming around to the PRs I owe you.

[cr1901]

@ThomasWaldmann Addressed your feedback and added a test. I am marking as ready for review.

[ThomasWaldmann]

Thanks! master branch needs checking, the hardlinks stuff is different (better) there.