compact: suggest a full check --repair for unindexed pack bytes, fixes #10429

[mr-raj12]

Description

Merge before #10445, which is rebased on this branch.

The info message borg compact logs for pack bytes no index entry covers pointed to #8572, which is closed. These bytes can be reclaimed today: a full borg check --repair rebuilds the chunk index from all packs, and the next borg compact reclaims the unused objects.

• compact: the message now says to run borg check --repair (without --repository-only), then borg compact. --repository-only rebuilds the index only if it is corrupt, so it does not index these objects. The borg compact --help text says the same.
• The remaining to pack or not to pack ... #8572 references (check --help text, repository check error message, comments) now point to borg2 check: implement repository repair (index and pack rebuild) #10026, which tracks repository repair and corrupt-pack salvage.
• test: test_compact_reclaims_indexed_waste_only asserts the new message.

Reproduced with a 2.5 GB borg create killed with kill -9:

step	repo size	message
borg compact -v	535M	shown
borg check --repair --repository-only, borg compact	535M	shown
borg check --repair, borg compact	20K	not shown
borg check	rc=0

Objects that fail authentication during the index rebuild stay unindexed, so compact keeps reporting their bytes after a repair.

fixes #10429

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (8274470) to head (8910d40).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10443   +/-   ##
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  Coverage   88.73%   88.73%           
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  Files         103      103           
  Lines       19387    19387           
  Branches     3023     3023           
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  Hits        17204    17204           
  Misses       1514     1514           
  Partials      669      669           

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