feat: switch from using the os/exec package to using mvdan.sh for git operations

[ga-paul-t]

closes #308

[codecov-commenter]

Codecov Report

Merging #315 (37defa1) into main (3d4ed26) will decrease coverage by 0.15%.
The diff coverage is 80.76%.

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
- Coverage   82.74%   82.60%   -0.15%     
==========================================
  Files          43       43              
  Lines        2226     2242      +16     
==========================================
+ Hits         1842     1852      +10     
- Misses        306      310       +4     
- Partials       78       80       +2     

Impacted Files	Coverage Δ
internal/git/utils.go	72.99% <79.16%> (-0.53%)	⬇️
internal/git/testing.go	81.40% <100.00%> (ø)

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