feat: restrict gitx execution to git repositories

[Divy13ansh]

What this PR does

• Adds a check at startup to ensure gitx only runs inside a Git repository.
• If run outside a repo, prints "Error: not a git repository." and exits with code 1.

Screenshots

Outside a Git repository

Inside a Git repository

Works normally

Fixes #22

[bakayu]

The CI pipeline failed-

Error: cmd/gitx/main.go:36:10: ST1005: error strings should not be capitalized (staticcheck)
  		return fmt.Errorf("Error: not a git repository")
  		       ^
  1 issues:
  * staticcheck: 1
  
  Error: issues found

Edit: It was for a previous commit, the latest commit fixes this. Please ignore this!

[bakayu]

Thank you for the contribution!