feat: require sync

[mtth]

This is a prerequisite to improving the conflict/merge flow.

Also refactor to omit the gitpython dependency.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.79279% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.07%. Comparing base (f8d0b32) to head (a20cc4d).

Files with missing lines	Patch %	Lines
src/git_draft/drafter.py	86.36%	4 Missing and 2 partials ⚠️
src/git_draft/git.py	98.11%	1 Missing ⚠️
src/git_draft/toolbox.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   91.94%   92.07%   +0.13%     
==========================================
  Files           9       10       +1     
  Lines         608      656      +48     
  Branches       43       46       +3     
==========================================
+ Hits          559      604      +45     
- Misses         32       33       +1     
- Partials       17       19       +2     

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