check: summarize findings and repairs

[charmikadi]

Description

borg check already reports individual problems as it finds them, but it was hard to tell at a glance how many issues there were overall, or how many --repair actually fixed.
This adds a short summary at the end of the check:

• for archive consistency checks, count problems found and repairs done
• for repository checks, include the error count in the finish message (repo repair still isn't implemented)
  Also updated a few repair tests to cover the new summary lines.

Fixes #2397

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.22%. Comparing base (6854dca) to head (14337fe).
⚠️ Report is 4 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	69.69%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9891      +/-   ##
==========================================
- Coverage   85.24%   85.22%   -0.03%     
==========================================
  Files          93       93              
  Lines       15570    15595      +25     
  Branches     2357     2365       +8     
==========================================
+ Hits        13273    13291      +18     
- Misses       1598     1604       +6     
- Partials      699      700       +1     

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[ThomasWaldmann]

@charmikadi Thanks for helping with this!

Just be aware that @mr-raj12 is currently working on packs and that might need bigger changes in borg check also.