stats: do not count data volume twice when checkpointing, fixes #3224

[ThomasWaldmann]

No description provided.

[codecov-io]

Codecov Report

Merging #3282 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3282      +/-   ##
==========================================
- Coverage   85.27%   85.23%   -0.04%     
==========================================
  Files          36       36              
  Lines        9003     9000       -3     
  Branches     1489     1488       -1     
==========================================
- Hits         7677     7671       -6     
- Misses        915      917       +2     
- Partials      411      412       +1

Impacted Files	Coverage Δ
src/borg/archive.py	83.51% <0%> (+0.15%)	⬆️
src/borg/archiver.py	87.17% <0%> (-0.4%)	⬇️
src/borg/cache.py	86.17% <0%> (+0.13%)	⬆️

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