Replace GitPython implementation with pygit2 one

[NotTheEvilOne]

What this PR does / why we need it:
This PR replaces GitPython implementation with pygit2. This allows proper handling of Git checkouts.

Which issue(s) this PR fixes:
Fixes #186

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.98%. Comparing base (c67c28b) to head (f0f6069).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/git/repository.py	90.32%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
- Coverage   91.08%   90.98%   -0.11%     
==========================================
  Files          27       27              
  Lines        1234     1264      +30     
==========================================
+ Hits         1124     1150      +26     
- Misses        110      114       +4     

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[yeoldegrove]

Not sure why you rename the whole class again.
Other than that, moving to pygit2 is fine.