feat: verify git repository before running git diff

[VarusHsu]

Previously, running git diff outside of a Git repository could cause the process to exit with code 137.

This PR adds a safety check to verify that the current working directory is inside a valid Git repository before executing git diff, preventing unexpected crashes and improving overall robustness.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 7.49%. Comparing base (0125d86) to head (b67165f).
⚠️ Report is 216 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/hepler.go	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0125d86) and HEAD (b67165f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (0125d86)	HEAD (b67165f)
	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #252       +/-   ##
==========================================
- Coverage   21.96%   7.49%   -14.48%     
==========================================
  Files           8      33       +25     
  Lines         346    1535     +1189     
==========================================
+ Hits           76     115       +39     
- Misses        264    1411     +1147     
- Partials        6       9        +3     

Flag	Coverage Δ
go-1.24	7.49% <77.77%> (?)
go-1.25	7.49% <77.77%> (?)
macos-latest	7.49% <77.77%> (?)
ubuntu-latest	7.49% <77.77%> (?)

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[appleboy]

@VarusHsu Thanks for your contribution. I have created another PR to fix the issue.