refactor(git): use go-git instead of shelled out commands

[andrewrynhard]

No description provided.

[codecov]

Codecov Report

Merging #14 into master will increase coverage by 11.99%.
The diff coverage is 53.48%.

@@             Coverage Diff             @@
##           master      #14       +/-   ##
===========================================
+ Coverage    8.13%   20.12%   +11.99%     
===========================================
  Files           2        2               
  Lines         123      164       +41     
===========================================
+ Hits           10       33       +23     
- Misses        110      119        +9     
- Partials        3       12        +9

Impacted Files	Coverage Δ
conform/enforce.go	22.6% <53.48%> (+13.07%)	⬆️

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