Add Unit Tests for Git Analyzer

Write unit tests for the internal/git/analyzer.go file to ensure commit analysis works correctly.

Tasks:

• Create analyzer_test.go file
• Write test for parsing commit messages
• Write test for calculating statistics (additions, deletions)
• Write test for filtering commits by date range
• Write test for filtering commits by author
• Achieve at least 80% code coverage

Acceptance Criteria:

• All tests pass with go test ./internal/git/...
• Code coverage is at least 80%
• Tests include edge cases (empty repos, no commits, etc.)

Resources:

• Go Testing: https://go.dev/doc/tutorial/add-a-test
• Table-driven tests: https://go.dev/wiki/TableDrivenTests