test: add more unit tests for the git comment

[LinuxSuRen]

No description provided.

[codecov]

Codecov Report

Merging #47 (84fe409) into master (04e31be) will increase coverage by 2.47%.
The diff coverage is 39.13%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   35.29%   37.77%   +2.47%     
==========================================
  Files           7        8       +1     
  Lines         391      413      +22     
==========================================
+ Hits          138      156      +18     
- Misses        251      255       +4     
  Partials        2        2              

Flag	Coverage Δ
unittests	37.77% <39.13%> (+2.47%)	⬆️

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Files Changed	Coverage Δ
cmd/pull_request.go	41.02% <25.00%> (+2.31%)	⬆️
pkg/git.go	10.71% <42.85%> (+2.74%)	⬆️
pkg/error.go	100.00% <100.00%> (ø)