compact: support --dry-run (do nothing), fixes #8300

[ThomasWaldmann]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.83%. Comparing base (694fa93) to head (1559a1e).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/archiver/compact_cmd.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8613      +/-   ##
==========================================
- Coverage   81.84%   81.83%   -0.01%     
==========================================
  Files          74       74              
  Lines       13311    13319       +8     
  Branches     1960     1963       +3     
==========================================
+ Hits        10894    10900       +6     
  Misses       1755     1755              
- Partials      662      664       +2     

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