bugfix: remove superfluous repository.list() call

[ThomasWaldmann]

Because it ended the loop only when .list() returned an empty result, this always needed one call more than necessary.

We can also detect that we are finished, if .list() returns less than the limit we gave to it.

Also: reduce code duplication by using repo_lister func.

[codecov]

Codecov Report

Attention: Patch coverage is 51.21951% with 20 lines in your changes missing coverage. Please review.

Project coverage is 81.54%. Comparing base (36e3d63) to head (1436bbb).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/archiver/debug_cmd.py	18.18%	18 Missing ⚠️
src/borg/repository.py	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8411   +/-   ##
=======================================
  Coverage   81.54%   81.54%           
=======================================
  Files          70       70           
  Lines       12791    12775   -16     
  Branches     2318     2317    -1     
=======================================
- Hits        10431    10418   -13     
+ Misses       1703     1699    -4     
- Partials      657      658    +1     

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