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

[ThomasWaldmann]

Sole purpose of this is to simplify scripting.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.60%. Comparing base (2230932) to head (09745d2).
Report is 14 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/archiver.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8596      +/-   ##
=============================================
+ Coverage      82.24%   82.60%   +0.35%     
=============================================
  Files             38       38              
  Lines          11076    11083       +7     
  Branches        2130     1715     -415     
=============================================
+ Hits            9110     9155      +45     
+ Misses          1390     1372      -18     
+ Partials         576      556      -20     

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