Safely log git failures in a special log file

[TimoWilken]

This is useful for CI, as we can show this log to the user if it exists, to indicate build failures due to failed git commands. The log file should not expose secrets to users, as it only contains the package name, git command and its exit code.

[codecov-commenter]

Codecov Report

Merging #784 (b22ef04) into master (b78f2b6) will increase coverage by 4.93%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
+ Coverage   85.19%   90.13%   +4.93%     
==========================================
  Files          30       15      -15     
  Lines        3027      973    -2054     
==========================================
- Hits         2579      877    -1702     
+ Misses        448       96     -352     

Impacted Files	Coverage Δ
tests/test_build.py	93.33% <100.00%> (-2.38%)	:arrow_down:
tests/test_workarea.py	92.42% <100.00%> (-3.23%)	:arrow_down:
tests/test_args.py	88.67% <0.00%> (-9.51%)	:arrow_down:
tests/test_deps.py	83.78% <0.00%> (-8.33%)	:arrow_down:
tests/test_doctor.py	87.30% <0.00%> (-8.02%)	:arrow_down:
tests/test_cmd.py	91.17% <0.00%> (-6.05%)	:arrow_down:
tests/test_init.py	89.09% <0.00%> (-5.65%)	:arrow_down:
tests/test_log.py	92.50% <0.00%> (-5.12%)	:arrow_down:
tests/test_clean.py	94.00% <0.00%> (-4.08%)	:arrow_down:
... and 22 more

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[TimoWilken]

Tested in a CI builder and working as far as I can tell.