Optimize List() pipeline

[middelink]

... I wanted to use contexts here too, but that requires too much plumbing right now.

[codecov-io]

Codecov Report

Merging #867 into master will decrease coverage by -0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #867      +/-   ##
==========================================
- Coverage   53.54%   53.44%   -0.11%     
==========================================
  Files          92       92              
  Lines        7636     7610      -26     
==========================================
- Hits         4089     4067      -22     
+ Misses       2931     2928       -3     
+ Partials      616      615       -1

Impacted Files	Coverage Δ
src/restic/mock/repository.go	57.05% <0%> (-2.42%)	❌
src/restic/archiver/archiver.go	64.96% <0%> (+0.22%)	✅

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[fd0]

Ok, that code was probably a bit too complicated. Cancelling the sending is handled by backend.List(), so it's fine.