git: change default add behavior to add(force=False)

[pmrowla]

NOTE: this is a breaking change and requires scmrepo major version bump

Closes #123

[codecov-commenter]

Codecov Report

Patch coverage: 97.50% and project coverage change: +0.08 🎉

Comparison is base (3dc4975) 81.01% compared to head (c3b4cc2) 81.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   81.01%   81.09%   +0.08%     
==========================================
  Files          27       27              
  Lines        3955     3978      +23     
  Branches      697      704       +7     
==========================================
+ Hits         3204     3226      +22     
  Misses        631      631              
- Partials      120      121       +1     

Impacted Files	Coverage Δ
src/scmrepo/git/backend/gitpython.py	65.00% <87.50%> (+0.20%)	⬆️
src/scmrepo/git/backend/base.py	80.88% <100.00%> (ø)
src/scmrepo/git/backend/dulwich/__init__.py	81.55% <100.00%> (+0.24%)	⬆️
src/scmrepo/git/backend/pygit2/__init__.py	78.03% <100.00%> (ø)
tests/test_git.py	100.00% <100.00%> (ø)

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