feat(shorebird_cli): create Git wrapper

[felangel]

Description

• feat(shorebird_cli): create Git wrapper (part of feat: shorebird should support switching flutter versions #1037)

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

Patch coverage: 90.90% and project coverage change: -0.04% ⚠️

Comparison is base (2c341b4) 100.00% compared to head (ad8d091) 99.96%.

❗ Current head ad8d091 differs from pull request most recent head fe2c10c. Consider uploading reports for the commit fe2c10c to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #1048      +/-   ##
===========================================
- Coverage   100.00%   99.96%   -0.04%     
===========================================
  Files           83       84       +1     
  Lines         3127     3138      +11     
===========================================
+ Hits          3127     3137      +10     
- Misses           0        1       +1     

Files Changed	Coverage Δ
packages/shorebird_cli/lib/src/git.dart	90.90% <90.90%> (ø)

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