Warn, but do not fail, when `git gc` returns 128

[jwodder]

Closes #191.

Note that the git gc calls cannot be centralized at the moment, as one is async and the other isn't.

[codecov[bot]]

Codecov Report

Merging #195 (977035e) into draft (a6d6de0) will decrease coverage by 0.88%. The diff coverage is 20.83%.

@@            Coverage Diff             @@
##            draft     #195      +/-   ##
==========================================
- Coverage   78.31%   77.43%   -0.89%     
==========================================
  Files           8        8              
  Lines        1416     1436      +20     
  Branches      228      231       +3     
==========================================
+ Hits         1109     1112       +3     
- Misses        232      249      +17     
  Partials       75       75              

Impacted Files	Coverage Δ
tools/backups2datalad/__main__.py	54.54% <0.00%> (-3.67%)	:arrow_down:
tools/backups2datalad/datasetter.py	72.43% <33.33%> (-0.95%)	:arrow_down:
tools/backups2datalad/zarr.py	79.92% <42.85%> (-1.03%)	:arrow_down:

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[yarikoptic]

coolio , thank you @jwodder !