fix: make git-commit promotion step check for diffs before committing

[krancour]

This logic was present in the legacy promotion mechanisms and was overlooked in the git-commit directive.

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[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 51.09%. Comparing base (16e4674) to head (4cea0c7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/directives/git_commiter.go	28.57%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2590      +/-   ##
==========================================
- Coverage   51.10%   51.09%   -0.01%     
==========================================
  Files         282      282              
  Lines       20945    20950       +5     
==========================================
+ Hits        10703    10705       +2     
- Misses       9578     9580       +2     
- Partials      664      665       +1     

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