check: report missing chunks grouped as chunk -> files -> archives, #9218

[mr-raj12]

Fixes #9218.

borg check logged one line per (archive, file, chunk) for each missing file chunk. Now it groups by chunk:

The following chunks are missing in the repository:
- Chunk a8675a3a8a...95dce, 553,688 bytes
    - var/lib/flatpak/repo/objects/2c/6265...52be.file: archive-2022-03-31, archive-2022-04-29
    - var/lib/flatpak/runtime/.../bin/gpgsm: archive-2022-03-31

For each missing chunk you see its size, the files that reference it, and the archives each file is in. The old per-chunk line (with byte offsets) is now logged at debug level.

Tests in check_cmd_test.py updated to the new format.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.74%. Comparing base (a50eb6f) to head (716a4ab).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9965      +/-   ##
==========================================
+ Coverage   85.70%   85.74%   +0.04%     
==========================================
  Files          95       95              
  Lines       16815    16859      +44     
  Branches     2577     2584       +7     
==========================================
+ Hits        14411    14456      +45     
  Misses       1668     1668              
+ Partials      736      735       -1     

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