Support repositories in fish tab completion (#5256)

[SanskritFritz]

As requested, the 1.1 backport. I used the squashed commit you performed in #5256

git checkout 1.1-maint
git cherry-pick 8b30a76ae2e5c56e5cbf16ae3e389a29dd66be85

[codecov-commenter]

Codecov Report

Merging #5361 into 1.1-maint will decrease coverage by 0.00%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           1.1-maint    #5361      +/-   ##
=============================================
- Coverage      80.12%   80.11%   -0.01%     
=============================================
  Files             27       27              
  Lines          10396    10396              
  Branches        1829     1829              
=============================================
- Hits            8330     8329       -1     
+ Misses          1555     1554       -1     
- Partials         511      513       +2     

Impacted Files	Coverage Δ
src/borg/archive.py	82.38% <0.00%> (-0.15%)	⬇️
src/borg/helpers.py	87.34% <0.00%> (-0.08%)	⬇️
src/borg/repository.py	85.42% <0.00%> (+0.19%)	⬆️

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