refactor(git): extract _create_commit_cmd_string, better test coverage

[bearomorphism]

Description

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run poetry doc locally to ensure the documentation pages renders correctly

Expected Behavior

Steps to Test This Pull Request

Additional Context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.79%. Comparing base (856ca4f) to head (575ad78).
Report is 3 commits behind head on refactors.

Additional details and impacted files

@@            Coverage Diff             @@
##           refactors    #1442   +/-   ##
==========================================
  Coverage      97.78%   97.79%           
==========================================
  Files             57       57           
  Lines           2620     2625    +5     
==========================================
+ Hits            2562     2567    +5     
  Misses            58       58           

Flag	Coverage Δ
unittests	97.79% <100.00%> (+<0.01%)	⬆️

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[bearomorphism]

I think I encountered the same issue:

#740 (comment)